Treasure Photos are treasure maps in Polaroid- they show a place which holds a hidden item of value. They are also keys- just knowing where the treasure is doesn't do you any good unless you've already discovered the photo.
Note: Once you have discovered and studied the photo, you can safely discard it without losing access to the cache.
Photo # 1
The first photo is found in the sublevels of the Main House, in the same room where you find the Scorpion Key.
The photo shows a fireplace which should look very familiar- the hunting-trophy-filled Drawing Room you just passed through. Move close and follow the prompt to reveal a cache of steroids.
Hidden, unlabeled steroids in a madhouse filled with genetic monstrosities. Sounds good!
The second photo is found inside the covered walkway to the Greenhouse entrance.
It shows a toilet next to an ashtray overflowing with cigarette butts. There are a lot of toilets in RE:7, but this is clearly the one from Zoe's trailer.
Go to the trailer for a prompt to retrieve a few rounds of Mag 44 ammo.
The third and final photo is found inside the Storage Room off the Master Bedroom, on a red stool.
This photo breaks the pattern a bit by being a place you haven't been to yet- some shelving with a head of some sort (could be a mannequin, right?)
The treasure here is actually in Lucas' obstacle course, in the second room of the tripwire section. Move the mannequin's head to reveal the game's second and last Repair Kit.