This seems entirely strange. I've completed Ada's campaign 3 times, once of Normal and twice on Professional. Each time I got to the Puzzle Room, the image was always the Serpent.
Odd, I never knew it could be the fish or eagle. Just keep trying all images man!
Currently Playing: Battlefield 4 (Xbox One) | Skyrim (Xbox 360)
Upcoming: Zoo Tycoon & Halo: Spartan Assault