File:Warhol spuds.jpg

This image (or all images in this article or category) was uploaded in the JPEG format. However, in this case, this indicates that someone was a retard. As anyone with basic knowledge of computer image technology (say, a year of applied computer-graphics courses at the undergraduate level) knows, JPEGs suck for diagrams and most screenshots. This is because JPEGs use what is known as lossy compression, which basically means "screw up photos all you want, humans are too blind to notice". But if you do that for solid-colored text on a solid-colored background, you know what? It looks like crap, and is bigger than an equivalent PNG image as well. And the worst part is, you've already screwed it up, so converting it now won't do anything. This image needs to be replaced by a newly-created PNG or SVG image.

So, photographs: JPEG. Screenshots of fancy 3D computer games: PNG. Diagrams: SVG, maybe, if you have something that can do it. Anything else: PNG. If you're not sure: PNG, because PNG is lossless and if you upload a 750 KB PNG when you should have uploaded a 10 KB JPEG that would look indistinguishable, someone can fix your mistake by just converting to JPEG. Now you know.