Unlocks after beating chapter 6 for the first time, either The Chest or The Dark Room.
First you have to beat either path of chapter 5 five times to unlock the corresponding photo, Polaroid for beating Isaac 5 times, Negative for beating Satan 5 times. Then on another playthrough, after you beat Mom, you will be able to pick one of them. If you pick the Polaroid, go fight Isaac again, if you pick the Negative, fight Satan. Whichever way, after you beat them while carrying the proper photo, you will go to chapter 6. After beating either one for the first time, a message will appear saying "The angels are waiting"