After completing the main story, you will have photographed all but one mural.
For the final mural, enter the glacial pathway from the sea people's village. Go down one layer to the room with the large mirror and then enter the cave to the left.
Solve the puzzle again (you did it before as part of a main story quest) and take a photo of the mural.
The achievement unlocks once you get back to the boat when Dr. Bacon drives up and talks to you.