--but make sure that those with mod-powers can't be ignored!

Customs would definitely have to have a size limitation, and perhaps no animated GIF's (but stills being OK)? Maybe if bandwidth is an issue they can only be uploaded from other places (photobucket, etc.) ... although that may bring up interesting problems if people use images linked to sites that forbid that practice. And lets not forget about people slipping porno avatars past the spotters.