In addition to evidence required to identify the ghost in your journal, you can take several different photos in Phasmophobia that will provide you will a little extra cash at the end of an assignment. In part, this is up to random luck and the type of ghost. However, some maps do have specific items on them that are worth a little cash from photos. The Photo Camera itself can be the most profitable part of an assignment other than completing the main objective.
At this time it seems that Photos are a bit buggy and sometimes they count and sometimes they just don't. Here is a list of the things that can count for a photo credit:
Ouija Board before using it
Ouija Board while the ghost is answering
Phone while it's ringing or just after
Voodoo Dolls in the Farm Houses
Dirty Water in a Sink
A Dead Body
The EMF Reader showing EMF 5
Writing in the Ghost Writing Book
The Microwave after it is used
The car alarm
The cell phone
Objects the ghost moves or throws
Keep in mind that this list is probably not complete. And remember it's still buggy. On one assignment I get the Ouja Board and the Ouija Board Interaction and on the next assignment it doesn't register. The same goes for Fingerprints sometimes and, well, everything, really.
Another thing to keep in mind is the Phantom. If you take a picture of the ghost and it instantly dissapears, check your journal as soon as it is safe to do so. If you have an empty photo that qualifies as a photo of the Ghost it is almost certainly a Phantom.
It is also normal for Photos taken by another player to show up empty. If you take the photo you will see the ghost or bone or whatever. If someone else takes it you probably won't see it. Take my screenshot below as an example: I took the Ouija Photos and my friend took the Bone photo.