MR provides a selection of 63 avatars that could be used over and over again by multiple people. I can understand why you might be upset (and it is a wonderful avatar), but I am not sure that there can be a rule about using an image like this one. I, on the other hand, created my avatar and actually own the image. I can't imagine that anyone wants a picture of my foot, of course.
There is an interesting article on Wired about Tux, who owns it, and its history. See:
http://www.wired.com/culture/lifesty.../2001/03/42209