Thumbnails of many images on Commons and English Wikipedia are appearing grainy, because the appropriately sized copies are not being generated. MediaWiki then upscales a low-res version, with hideous results.
Affected images appear to mostly be large JPEGs with a portrait orientation (long Y axis).
See these pages for examples:
https://commons.wikimedia.org/wiki/Commons:Graphics_village_pump
https://test.wikipedia.org/wiki/User:Guanaco/sandbox
Also affects local uploads:
https://en.wikipedia.org/wiki/File:Green_and_golden_Butterfly_copy.jpg