pngcrush finds that a number of core/resources/lib/oojs-ui/themes/mediawiki/images/*/*png files can be reduced in size.
I would have submitted a patch against OOjs but oojs/ui/src/themes/ contains no PNG files and docs don't mention what creates them. Please adjust your PNG-creation methods before throwing those files in resources/lib/, thanks.
Numbers:
$ find resources/lib/ -name '*png' -exec pngcrush -force -brute {} {}.out \; 2>&1 | grep -B 2 "filesize reduction" for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-down.png.out (8.43% critical chunk reduction) (6.90% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-rtl-invert.png.out (11.46% critical chunk reduction) (9.61% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/alert-invert.png.out (13.66% critical chunk reduction) (11.74% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-up.png.out (10.12% critical chunk reduction) (8.29% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-ltr-invert.png.out (4.65% critical chunk reduction) (3.83% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-down-invert.png.out (2.76% critical chunk reduction) (2.29% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-ltr.png.out (12.94% critical chunk reduction) (10.63% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/required.png.out (6.86% critical chunk reduction) (5.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/alert.png.out (9.18% critical chunk reduction) (7.73% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-up-invert.png.out (8.33% critical chunk reduction) (6.99% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/arrow-rtl.png.out (13.22% critical chunk reduction) (10.90% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/indicators/required-invert.png.out (6.11% critical chunk reduction) (5.26% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/search-invert.png.out (7.88% critical chunk reduction) (6.83% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/move-rtl-invert.png.out (6.52% critical chunk reduction) (5.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/arched-arrow-rtl-invert.png.out (12.40% critical chunk reduction) (10.85% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/expand-invert.png.out (1.14% critical chunk reduction) (0.94% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/settings.png.out (11.04% critical chunk reduction) (8.90% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/arched-arrow-ltr.png.out (8.23% critical chunk reduction) (7.09% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/remove.png.out (15.56% critical chunk reduction) (12.21% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/circle-invert.png.out (12.66% critical chunk reduction) (10.95% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/circle.png.out (8.70% critical chunk reduction) (7.38% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/alert-invert.png.out (8.10% critical chunk reduction) (7.46% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/add-constructive.png.out (16.52% critical chunk reduction) (12.50% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/move-ltr-invert.png.out (4.26% critical chunk reduction) (3.56% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/tag-invert.png.out (10.08% critical chunk reduction) (8.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/advanced.png.out (12.79% critical chunk reduction) (11.78% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/picture.png.out (15.34% critical chunk reduction) (13.71% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/search.png.out (10.05% critical chunk reduction) (8.54% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/link.png.out (8.36% critical chunk reduction) (7.37% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/add.png.out (13.08% critical chunk reduction) (9.72% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/settings-invert.png.out (12.73% critical chunk reduction) (10.40% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/move-ltr.png.out (9.34% critical chunk reduction) (7.76% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/close-invert.png.out (3.96% critical chunk reduction) (3.41% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/check-constructive.png.out (26.25% critical chunk reduction) (23.53% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/window.png.out (7.01% critical chunk reduction) (5.98% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/info-invert.png.out (7.94% critical chunk reduction) (7.16% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/collapse.png.out (8.82% critical chunk reduction) (7.25% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/arched-arrow-rtl.png.out (8.97% critical chunk reduction) (7.69% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/remove-destructive.png.out (19.74% critical chunk reduction) (15.87% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/close.png.out (5.74% critical chunk reduction) (4.88% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/code-invert.png.out (7.69% critical chunk reduction) (6.73% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/help.png.out (14.02% critical chunk reduction) (13.04% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/arched-arrow-ltr-invert.png.out (11.24% critical chunk reduction) (9.87% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/tag.png.out (7.11% critical chunk reduction) (6.11% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/check-invert.png.out (8.48% critical chunk reduction) (7.28% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/alert.png.out (9.09% critical chunk reduction) (8.31% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/code.png.out (9.36% critical chunk reduction) (8.09% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/menu-invert.png.out (15.54% critical chunk reduction) (12.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/remove-invert.png.out (20.14% critical chunk reduction) (16.02% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/advanced-invert.png.out (14.20% critical chunk reduction) (13.19% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/add-invert.png.out (14.41% critical chunk reduction) (10.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/comment.png.out (12.64% critical chunk reduction) (10.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/circle-constructive.png.out (19.80% critical chunk reduction) (17.61% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/clear.png.out (3.82% critical chunk reduction) (3.42% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/picture-invert.png.out (13.93% critical chunk reduction) (12.63% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/link-invert.png.out (3.69% critical chunk reduction) (3.28% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/help-invert.png.out (10.25% critical chunk reduction) (9.62% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/menu.png.out (12.59% critical chunk reduction) (9.88% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/comment-invert.png.out (16.84% critical chunk reduction) (14.16% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/window-invert.png.out (8.90% critical chunk reduction) (7.69% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/check.png.out (8.08% critical chunk reduction) (6.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/move-rtl.png.out (9.60% critical chunk reduction) (7.94% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/clear-invert.png.out (0.29% critical chunk reduction) (0.26% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/icons/info.png.out (11.34% critical chunk reduction) (10.06% filesize reduction) -- for output to resources/lib/oojs-ui/themes/mediawiki/images/textures/transparency.png.out (16.67% critical chunk reduction) (12.41% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/arrow-down.png.out (8.33% critical chunk reduction) (6.51% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/arrow-up.png.out (8.21% critical chunk reduction) (6.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/arrow-ltr.png.out (9.30% critical chunk reduction) (7.23% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/required.png.out (6.86% critical chunk reduction) (5.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/alert.png.out (9.18% critical chunk reduction) (7.73% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/indicators/arrow-rtl.png.out (8.21% critical chunk reduction) (6.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/settings.png.out (11.04% critical chunk reduction) (8.90% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/arched-arrow-ltr.png.out (8.23% critical chunk reduction) (7.09% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/remove.png.out (15.56% critical chunk reduction) (12.21% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/advanced.png.out (12.79% critical chunk reduction) (11.78% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/picture.png.out (15.34% critical chunk reduction) (13.71% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/search.png.out (10.05% critical chunk reduction) (8.54% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/link.png.out (8.36% critical chunk reduction) (7.37% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/add.png.out (13.08% critical chunk reduction) (9.72% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/move-ltr.png.out (9.34% critical chunk reduction) (7.76% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/window.png.out (7.01% critical chunk reduction) (5.98% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/collapse.png.out (8.82% critical chunk reduction) (7.25% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/arched-arrow-rtl.png.out (8.97% critical chunk reduction) (7.69% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/close.png.out (5.74% critical chunk reduction) (4.88% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/help.png.out (14.02% critical chunk reduction) (13.04% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/tag.png.out (7.11% critical chunk reduction) (6.11% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/alert.png.out (9.09% critical chunk reduction) (8.31% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/code.png.out (9.36% critical chunk reduction) (8.09% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/comment.png.out (12.64% critical chunk reduction) (10.43% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/clear.png.out (3.82% critical chunk reduction) (3.42% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/menu.png.out (12.59% critical chunk reduction) (9.88% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/check.png.out (8.08% critical chunk reduction) (6.81% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/move-rtl.png.out (9.60% critical chunk reduction) (7.94% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/icons/info.png.out (11.34% critical chunk reduction) (10.06% filesize reduction) -- for output to resources/lib/oojs-ui/themes/apex/images/textures/transparency.png.out (16.67% critical chunk reduction) (12.41% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_flat_75_ffffff_40x100.png.out (no critical chunk change) (47.12% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_glass_75_dadada_1x400.png.out (1.55% critical chunk reduction) (38.17% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_glass_55_fbf9ee_1x400.png.out (6.57% critical chunk reduction) (33.13% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_glass_75_e6e6e6_1x400.png.out (6.20% critical chunk reduction) (40.46% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-icons_222222_256x240.png.out (30.08% critical chunk reduction) (31.29% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_glass_65_ffffff_1x400.png.out (no critical chunk change) (47.34% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_highlight-soft_75_cccccc_1x100.png.out (16.33% critical chunk reduction) (43.57% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-icons_cd0a0a_256x240.png.out (2.76% critical chunk reduction) (5.56% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-icons_454545_256x240.png.out (30.80% critical chunk reduction) (31.98% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-icons_888888_256x240.png.out (30.82% critical chunk reduction) (32.00% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_flat_0_aaaaaa_40x100.png.out (no critical chunk change) (46.23% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-bg_glass_95_fef1ec_1x400.png.out (4.10% critical chunk reduction) (31.93% filesize reduction) -- for output to resources/lib/jquery.ui/themes/smoothness/images/ui-icons_2e83ff_256x240.png.out (2.76% critical chunk reduction) (5.56% filesize reduction)