You can actually unlock all of the photos in the gallery in-game by progressing through the story and completing quests, not by losing. You can even unlock all of the pictures from the vampire route at a later point, even while on the Hunter route.
If you need help unlocking specific photos, I’ve written a full walkthrough that you can check out here: https://docs.google.com/document/d/1VbSILz60_Yc7NyU-DRv3mrX0FVua3WSBD9nlN0Bf5jk/edit
The only way to just unlock everything immediately is by using the cheat in the Dev Room, which is only accessible to game patrons.