I always feel like that the hotel and its forces were just so powerful, that once Jack died, he'd already sort of given his soul to the hotel, so they were sort of just able to suck him in and back in time and retroactively make it so he felt he'd been there all along.

That is another indication that Jack can shine, just like his son and the chef. Danny can see past events the Grady twin girls, just as Jack can see the past and communicate with people in the past, bartender, Grady, the woman in the bathtub, etc.

His picture on the wall is perhaps just a coincidence: that someone in the past looked like him. The film may even be implying that his lookalike could shine as well, and that there may have been many shiners throughout history. When Wendy sees ghosts in the hotel, she may be shining too.

The whole story is obviously a metaphor about the mental distress suffered by this family that manifests itself as ghostly visions and such. It has been done before in films, notably in Roman Polanski's repulsion, where a depressed woman sees all kinds of repulsive hallucinations.