Tests for hardcoded image sizes

[TobiGr]

It would be good to have some kind of automated check / test that checks the hard-coded values from time to time.

E.g. for the changes done in #1525 (comment)

[tomatotomata]

I was thinking of adding a focused regression test around the YouTube comment avatar fallback from #1525, asserting the fallback image dimensions and resolution metadata instead of duplicating the literals in another path. I would first check how extractor fixtures expose the parsed image list, then keep this limited to the new fallback behavior. Does that sound like the useful boundary for this test issue?