there is no definite answer to this question. It's whatever seems right to you.
People have their theories and opinions, so I'll give you mine, seeing as you're asking:
Jack Torrance was at the overlook hotel in 1921. That is what the photo shows, doesn't it? He's also "the centre of attention" because he's at the very front. This is also shown when he talks to Grady in the film, who says: "you're the important one".
This is the overlook Hotel "absorbing" or "shining" him in, and thus, he becomes a part of it. Just like all the other ghosts.
The film takes place in the 1970s, and the Jack who we see throughout is a DEMONIC VERSION of the Jack Torrance who was there in 1921. The same happened to Grady. He was at the Hotel in the past (1921): "I've always been here" and the hotel absorbed him in. And in the 1970s, a DEMONIC VERSION of him killed his wife and kids at the hotel.
The hotel is evil and it tries to "collect" people which it then uses to kill people. All the people you see in The Gold Room are its victims. The hotel tried to use this evil version of Jack Torrance to kill Wendy and Danny but they got away. However, the hotel did claim another victim, Dick Halloran who would undoubtedly become a part of the hotel forever.
The hotel represents hell. And that photo in 1921 shows Jack Torrance - wearing the same RED tuxedo that Lloyd the Bartender wore - and he's supposed to represent the Devil. And the people behind are all damned.