The new upload page is really great with the way it gives you exactly what you should post to get your pictures to appear. Also in the way it will tells you if your picture is formatted wrong or is to big.
Why not make (or add) to more sections. One that is specific for avatars. Like a "Avatars Only", that will make sure the size is correct. Also a section for signature pictures. Like, "Signature Pictures Only" that will also make sure that those pictures are the correct size. This will hopefully eliminate the problems that the mods are having enforcing the rules.
Just some thoughts....