The Photo Camera in Phasmophobia is often the most profitable piece of equipment in the game. Only Objective One, Identify the type of Ghost, produces more money than the Photo Camera. What counts as a Photo worth something, however, is another matter.
Using the Photo Camera in Phasmophobia may seem simple, but it can get weird. There are some bugs at this time with the Photo Camera, which can make it hard to determine what does and doesn't count as a photo.
For the most part, Photos of the Ghost, Dirty Water in a Sink, Ghost Writing in the Ghost Writing Book, and Fingerprints should count. You'll want to make sure you take the Photo up close and personal or it may not count correctly. Please keep in mind that this mechanic is unstable at best, so don't be too upset if your Photos don't get counted.
A comprehensive list of photo opportunities that can count for a cash bonus can be found here: Phasmophobia Photo Evidence.