Photos are taken during the freestyle section of songs, and then one during the finishing move. Playing a song in Perform It or Fitness mode will get you 17 photos per song. There are no freestyle sections in the Crew Challenges, so you'll only be getting 1 photo per song. There are over 40 songs in the game, so the best way to unlock this is to practice learning songs on Hard in Perform It mode in preparation for the Crew Challenges. That way, you get a lot more photos per song than practicing in Crew Challenge mode.