Or maybe there's still hope that someone out there will unearth higher resolution shots somewhere? It continuously surprises me when such huge films have imperfect resources. Have you tried contacting any Superman fan sites? Perhaps someone has a collection of old press photos or lobby cards they can scan?
EDIT: Oops, I forgot that there was a fourth movie, plus Superman Returns. A 4-in-1 cover might still work really nicely for the Reeves movies. Or, maybe the second suggestion of contacting fan sites will be a good direction.
