I think avatar pictures have the same range as our login names. Some people use photos of themselves like some posters use their own name. And on the other side of things, some people really identify with their avatar and want to be called by their login name preferably. And the rest just sits in between...
Personally, I just chose this nick because my real name was taken in some RPG forum (probably by myself, with an old unreachable email adress) and ended up using a random NPC name. For some kind of reason, I re-used that character here. I probably takes just a few more people speaking of themselves as their pseudonyms (I don't want to point fingers...) to go back to a permutation of my real name...