These are movie stills. The Star Wars 'Faces' release has Luke, Leia, Han, etc on the back, but only in movie stills as well. I'm not a copyright expert, but I'm guessing there is a legal difference between showing stills from the film on the back, and using photo images on the front. Of the people on the Superman III cover, Christopher Reeves is basically too important not to include,especially when you are talking in terms of the entire film series, and the others except Robert Vaughn aren't that famous. Richard Pryor's estate probably wanted quite a bit for his image, so WB decided that it wasn't worth it for only one film.