Quickie:
Kubrick gives one explanation below:
From wikipedia:
The photograph
At the end of the film, the camera moves slowly towards a wall in the Overlook and a 1921 photograph, revealed to include Jack seen at the middle of a 1921 party. In an interview with Michel Ciment, Kubrick said that the photograph suggests that Jack was a reincarnation of an earlier official at the hotel. This has not stopped alternative readings, such as that Jack has been "absorbed" into the Overlook Hotel. Film critic Jonathan Romney, while acknowledging the absorption theory, wrote:
As the ghostly butler Grady (Philip Stone) tells him during their chilling confrontation in the men's toilet, 'You're the caretaker, sir. You've always been the caretaker.' Perhaps in some earlier incarnation Jack really was around in 1921, and it's his present-day self that is the shadow, the phantom photographic copy. But if his picture has been there all along, why has no one noticed it? After all, it's right at the center of the central picture on the wall, and the Torrances have had a painfully drawn-out winter of mind-numbing leisure in which to inspect every corner of the place. Is it just that, like Poe's purloined letter, the thing in plain sight is the last thing you see? When you do see it, the effect is so unsettling because you realise the unthinkable was there under your nose – overlooked – the whole time
But it can be up to you to take what you will from it.
It is also a very commonly asked question:
https://screenrant.com/shining-movie-ending-meaning-photo-book-explained/
Kubrick himself has said the photo actually suggests Jack being a reincarnation of an earlier official at the hotel. This makes sense when going back to his conversation with Grady in the bathroom, where the butler tells Jack that he has “always been the caretaker.” It also fits with the role of both Grady characters mentioned in the film: the past caretaker and the ghost. The ghost is Delbert Grady, and the past caretaker was Charles Grady. The latter is the one that Jack says he saw in the newspaper (and the one who killed his family in the hotel), and thus the reincarnation of Delbert Grady.
https://collider.com/the-shining-ending-explained/
https://the-take.com/read/at-the-end-of-the-shining-why-is-jack-in-the-photo-of-the-overlooks-july-4th-1921-party
(The photograph, by the way, is an actual photo from 1921 with Jack’s head airbrushed onto another fellow’s body.)
Jack: I fell in love with it right away. When I came up here for my interview, it was as though I'd been here before. I mean, we all have moments of déjà vu, but this was ridiculous. It was almost as though I knew what was going to be around every corner.
https://faroutmagazine.co.uk/20-years-of-bowling-for-columbine/
The absorption theory the OP mentions is just one of the more popular theories that have been around for a very long time.