Grady tells Jack, "You've always
been the caretaker." The picture at the end, then, implies the truth of this. For example, Jack's insanity - combined with his perpetual existence inside the hotel - manifests in various different stories (e.g. a wife, Wendy, and a son, Danny) but ultimately he has just "always" been part of the hotel.
In a sense, the whole movie is just a manifestation of Jack's insanity. Whether you're talking about the "sane" family life - the writer trying to make ends meet - or the guy who goes nuts with an axe, it's all inside Jack's crazy head.
This is just my opnion, of course.