It's a little annoying especially for black text/art, and it's not too difficult to have thumbnails be generated with a different background color

Take the "heyuri seal of epic win" for example - I only ever post the version with the pre-baked background color since it looks crappy with a black background, but it would be nicer for everyone wanting to use it (whether it's reposting it, using it in an edit, making stickers, etc.) if the transparent version was being posted instead

Hell, just look at the Heyuri logo I made that's currently on teh wiki's main page - I pre-baked the background so it didn't look liek crap in the thumbnail, and then whoever decided to make it the logo did a poor job of cutting it out, leaving some of the original background unremoved