Christina Wodtke suggests that one way to get people to upload meaningful avatars is to give them unrelated ones as a default. Her example shows an extremely beautiful woman (looks familiar but I am no celebrity expert) that doesn’t quite sell her point IMHO. She does suggest that an image like a cro-magnon might do it. The exact motivational image probably depends on your community; William Shatner, Hilary Clinton, Charles Manson or Hello Kitty all seem capable of pacifying and provoking potential users in the right setting.