Before you go and do the final puzzle:
*** Spoiler - click to reveal ***During the final day, when there's snow covering the ground outside, and you must go down to the cellar and do the puzzle with the boiler levers.
Make sure to fill your phone camera's memory. This means taking a total of 30 photos(and not deleting them). They can be of anything, just fill up all 30 slots.
The achievement unlocks when you complete the game.