Serenedipity, are you talking about those tiny "M's which appear beside their names ?
If so that's their title, they are all moderators and the M identifies them as what they are. When you see them, you can know that they are the mods of the forums. You can get one of the by becoming an moderator.
If you're talking about the little picture just beneath the names, you mean avatars.
I'll leave that for some one else to explain. !