This achievement is earned for finding optional photo #2. 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 #2
Outside of the girl's dorms, go to the right to the squirrel on the bench, rewind time, and capture it standing up near the beginning of the animation loop.
Collecting all 10 photos will also reward you with the following achievement: