Except it'd be crucial that screenshots are saved at 100% JPG quality then, to ensure we don't lose any information. And in mods, JPG should NEVER be used (I already loath fernland, despite never having used it, because it used the evil JPG). JPG is a lossly format, so it is very bad for contributory efforts like this, where we may often need to modify something, or expand on anothers work. JPG loses data each time it's saved, and that's simply no good. And at 100% quality, JPG doesn't save much over PNG. The problem with supporting JPG is that it can make an unknowning person make an entire mod with JPG images. And in most image editors, the default JPG quality is usually 85-90%, so you'll lose quality fast.
Filesize isn't every thing. Even for those who have terrible internet like me.