it's a a personal choice. everyone expreses themselves in different ways, and most members express themselves through their signatures and avatars. Using the photo as the basis for a person's personality is very shallow. It's a member's own discretion if s/he would use her/his own photo as an avatar. you can encourage others though. but to include it in the guidelines, i think not. why not talk and mingle with everyone so you'll be able to know their true personality than judging by their faces.