Photograpy challenges are marked on the map with an icon. However, several of the photo events will not be present until you've completed the main storyline. There are three types of photography events:
Investigator - Retrieved all parts of Detective's journal
These events will have the detective (apologizes, I forget her name) standing by her patrol car. An open laptop will be sitting on the hood of the car. You'll need to take a shot of the screen.
Special Agent - Retrieved all parts of Menken's journal
Two men in suits will be standing together. You'll need a picture of them.
Hunter of Killers - Cletus Cassidy Markers
You'll come upon a murder scene with an investigator and the letters "CK" written on the wall in blood. You'll need a shot of the letters.
For completing all of these, you'll receive the Reporter cheevo as well.