There aren't any specific guidelines on avatars; lots of people use game related images (mine is from a Crystal Eyes CD cover). No D:OS 2 avatars have been added to the forum yet (check under My Stuff, Edit Profile, then the avatar section at the bottom, for what is available). You could crop and resize a screenshot or artwork to within 120x120 resolution, if you can host it somewhere that allows hotlinking (linking directly to an image, or other file, from a different website; if your ISP provides webspace, that should work). Otherwise there are sites with various avatars that you can use.