My overall take is that Jack wasn’t a maniac, he was a creative writer who took the story he was told in the opening of the film and used it as inspiration to write a horror story (specifically for his wife, a lover of horror stories). He self-inserted himself in to the story. When we see him being crazy, they’re scenes from his book, not reality.
The photo is the part I don’t really understand, but I think it’s basically a nod to the idea that Jack wrote himself in to the story in place of the original killer, Grady.
Also, the ‘All work and no play makes Jack a dull boy’ scene is simply genius. The sheer amount of pages his wife goes through, showing how long he has been insane for is extremely unsettling and so well-executed.