I'll hafta disagree w/ u cc because every time I've noticed that people move up and down in the author list as the contribute more than someone else. the c-index may determine an author's position if there are two authors with the same amount of images though.
I used to think it was number of images too lightning... but it makes sense. The more pictures you have the more c-index you get. =) A more c-index based system would probably the author's c-index average.