Just as the title says really. Is there a preference to whether JPEG or PNG versions or images are uploaded? I do get the basic differences between them - JPEGs have a more compact file size, have little degradation in the quality and are good with showing a large spectrum of colors, but suffer from having a lossy format (which means each time you open the image it compresses and you lose a small amount of information). PNGs are clear, have a generally larger file size and are good for storing intricate details on simple pieces, like for line drawings and text. It is also works in a lossless format. I've heard color is more limited on a PNG though, but I can never seem to notice much of a difference.
I think PNG is more recommended for full scale artworks and large graphic files, which makes sense. But I'm not so sure about screen shots - some sources recommend JPEG files for those. I'm guessing it depends on the system the game is on and its resolution. Another thing to note is the conversion of images, as converting PNG-to-JPEG effects the quality negatively (and I think it's the same the other way around as well?).