I have a personal rule against putting up pictures of myself on the internet.. I just like the anonymous aspect, partly, but also partly because of security reasons. And hey! Avatars and images convey more meaning about yourself anyways, so why not make a use of the medium anyway?
There's no rule against using pictures of yourself, indeed we have a thread for people who want to post pictures of themselves, but imo using a picture of yourself is missing the opportunity to use an avatar that better represents who you are.