If Jack had 'always been the caretaker' then why...?

No idea. I love the movie, but this ending has never made sense to me. There is literally nothing in the film to explain it.

Also, what are all these wealthy people doing in a remote hotel in the mountains in the middle of the summer? Isn't this supposed to be some kind of ski resort?

A caretaker can put on his evening clothes and pay for a drink at the hotel bar, same as anyone!

FYI in the1920s it wasn't just rich people who wore tuxedos or fancy gowns to bars, it was anyone who could afford "evening dress". So what looks like a gathering of rich people to us was more likely to be middle-class hotel guests and maybe some local people, they're just more formally dressed than their modern equivalents would be.

There was a 1920's party when Jack saw Grady's ghost and they talked in the bathroom. Jack got absorbed into the hotel and is stuck in the 1920's era, I suppose.

Remember in the book/movie The Witches, the witches are able to lock children inside of paintings. And the children movie around to various spots within the painting, but no one ever sees them actually move.

Maybe the Overlook is kind of like that. Once it owns your soul, it can do whatever it wants with you. Including putting you in pictures of events that there's no way you ever would have been at, in various poses, and outfits. Basically you could walk by that picture every day, and every day it might be different, with different people, in different places.

I have been planning to post about the ending of this movie, whynotwriteme, but since you have done it first I'll answer your question instead. I think that Jack is the reincarnation of the original owner, not caretaker, of the hotel, hence his spotlight in the final image. He remembers the building from another lifetime, and Grady insists that Jack has always been the caretaker. In this instance, though, 'caretaker' is being used as an umbrella term to refer to someone who is simply in charge of The Overlook Hotel, not a specific position of employment. The ghostly party and picture of the ball are from 1921, which explains why Grady has no memory of attending to the facility, killing his family, or committing suicide in 1970. The older man claims to have always been at the location because he is permanently trapped in the time setting of the building's spirit. 1921 might be when the hotel reaches its successful peak, or perhaps Jack's previous self dies in that year before his property can overtake him. While Jack probably does not have identical features in both of his lives, his most current face will be the one in the last photograph if the hotel reclaims him. The novel and miniseries focus on the building's evil state trying to capture and use Danny. However, this film seems to say that The Overlook Hotel just wants to get the small boy out of the way so that it can get Jack back. It's reasonable that the resort would strive to reconnect to the man who has inadvertently fueled its power.

He was a charismatic guy. The other ghosts loved having him around.

It's not out of the realm of possibility that the staff of the overlook was invited to the party, it's not unheard of. As for the picture I am of the opinion that Jack is a reincarnation of the guy in the picture just as Charles Grady is the reincarnation of Delbert Grady. The Jack in the picture and Delbert Grady (along with many others) are cursed by the Indian Burial Ground to be reincarnated, find their way back to the hotel (hence the Deja Vu) and provide the hotel with a blood sacrifice (Grady's wife, daughters, Danny, Wendy).

It's possible that while the original Jack (from the photo) and Delbert Grady were alive Jack was the caretaker and Delbert Grady was a waiter/butler and Charles Grady was just lured back to the hotel by the caretaker job (which was originally Jack's back in the 1920's).

Also the Overlook is high in the Rocky's, very possible it was cold enough to ski (probably had near freezing temperatures year round).