Curious about this, I looked for the dictionary definition of Avatar:

Definition

As I'm sure we are not concerned with incarnations of Hindu dieties, then the 'jargon' meaning is an image representing a user.

As such, it should be a simple graphic reflecting your rpersonality or interest. I think that taking a photograph and reducing it to the standard avatar size defeats the purpose of the avatar. If you feel the need to post pictures of yourself or your models, I would suggest the gallery.