There are 30 Photo Investigations. These are extremely easy and quick to complete and there are only 3 types of photo challenges that you'll need to photograph and are associated with 3 other achievements.
1. "Special Agent" - there will be two guys in suits you'll need to photograph
2. "Investigator" - there will be a cop with a laptop, just photograph the laptop
3. "Hunter of Killers" - there will be a cop over a body, you'll need to find Cletus Kasady's initals "C.K." on a nearby wall to photograph