I never got a straight answer when I asked this either. All they said was that they were special and they were the only ones allowed to have them (not joking, they said that).
If I had to guess though, I'd say it might be because of certain users' connection speeds being low. So people coming to the board just to look up an answer to a problem they're having with ESF won't have to wait for a stream of images to load before they can do so (which also makes no sense because they could just use the search function without registering, and look up their answer that way, since we all know that every single question has been answered at one point or another).
Edit: Oh, and for those who don't understand that last bit, images (sigs and avatars) don't display to unregistered users.