This achievement is earned for finding optional photo #10. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional photos through collectibles mode. The following collectible is described and can be seen below:
] - Optional Photo #10
From where you enter the party, while standing near the diving board, look to the right out the window to spot the double moon. Take a picture of the double moon through the window at the top of the gymnasium.
Collecting all 10 photos will also reward you with the following achievement: