This isn't Wikipedia, and we have no intent of starting a userbox war or restricting what can go in it. Since this is a fan site and not a strict encyclopedia, we follow "netiquette" law, meaning copyrighted images may go in userboxes. We have no image restrictions, except to please try keep the image size limited to 45 pixels in height, no more, no less. This is just so that the userboxes fit well.
As for what kind of userboxes you can't make, just use common sense. No racism, porn, et cetera in userboxes obviously. You can make a userbox saying "X sucks!" as long as X isn't a person or belief (although fictional characters and Tom Cruise are exempt from this restriction). Other than that, you can create any userbox you want, from what your favorite sports team is to don't you hate your stupid 8:30 am lecture.