Thumbnails in popups don't appear in Opera 12.
@ovasileva I did some testing on Opera browsers, but concentrated on the more recent versions. I did a quick sampling of Opera versions on various Windows systems.
Opera versions 43 40 37 35 32 30 26 24 - Everything appears to fine
Opera versions 23 21 19 15 - Displays image alignment problem on some images
But not on others
Opera versions 12.12 12 - No image appears
Note that Opera 15 and later use the Blink rendering engine, same as Chrome etc. – so if you test on Chrome, testing on modern Opera is not really necessary. Opera up to version 12 used the Presto rendering engine (Opera 15 was effectively a total rewrite; they skipped 13 and 14). Many of its features haven't been re-implemented in the new one, so some folks still use the old browser.
I'm happy to help with Opera 12 support, mainly for nostalgia reasons ;) In my experience it does not require much effort, and often Opera 12 bugs help uncover other issues in the code (like in this task, where I fixed it by replacing some ugly hacks).
The version numbers don't really matter, since they changed the versioning scheme after 12. The latest update for Opera 12, Opera 12.18, was actually released in February 2016. And Opera 12.1+ is actually listed as supported at Grade A on the page you link, under "Browser support matrix".
The patch was in fact already merged, and the fix involved removing 30 lines of code and adding 6. This task is already fixed, I hope you don't want to revert it out of principle.
Given that PagePreviews work perfectly on Opera 12 now, it seems silly to drop support for it (I assume this means blacklisting it somehow). It would be reasonable to simply not officially support it, but then that is already what you do (I don't think anyone ever claimed that it's supported).
Using crossbrowsertesting I checked three versions of Opera 12 using the Mac OSX 10.11 setting, and according to the screencaps below page previews appear as expected.
Mac OSX 10.11 Opera 12.14
Mac OSX 10.11 Opera 12.12
Mac OSX 10.11 Opera 12.00