Once you've taken a picture of all the four Moon symbols, go to the secret room, and there'll be music playing on the record player. Pick up the golden disc, then go to the wall to the left and interact with the poster. The Moon symbol photos should also all be there.
Click a few times on the disc that's now on the moon poster to rotate it, and letters will appear on each of the photos. Put the photos in the right order to spell a word, and the achievement will unlock.
The word/solution is: