This achievement is earned for finding optional photo #9. 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 #9
At the party, inside the washroom, you'll be able to take a picture of Justin standing next to a wall with skeleton graffiti on it. Line up the photo so that it looks like the skeleton and the character are a person.
Collecting all 10 photos will also reward you with the following achievement: