http://esr.ibiblio.org/?p=1404#comment-242133
http://esr.ibiblio.org/?p=1404#comment-242156
Bottom line: libgif and libpng.
Worth noting that multiple years span the time between when Elber actually wrote giflib and when ESR added 2 functions and 3 utilities to it.
(Thanks for the answer though --- I think you're right).