Literally, of course, Nick, the narrator of The Great Gatsby, is neither a painter or photographer. I assume then that you are thinking figuratively, and asking whether Nick as narrator is more similar to a photographer or a painter.
Speaking simplistically, Nick is more of a painter than a photographer. Viewed simplistically, a photographer only captures what's present. A photographer doesn't comment on or interpret what he films (again, thinking simplistically). A photographer just takes pictures of reality.
A painter comments on, or filters, or interprets what he paints. The night sky reflects what's going on in van Gogh's mind in one of his most famous paintings, for instance. A painting is further removed from reality than a photo is.
This might represent Nick's narration in the novel. Nick is an unreliable narrator who reveals his prejudices and opinions throughout the novel. He may say that he doesn't think he is better than anyone else and that he doesn't judge people, but he actually does. Everything in the novel is filtered through Nick's eyes, and arranged in his mind.
Nick remembers Tom from college, and before he's even met Tom in the present of the novel he condemns him. He judges Jordan before even being introduced to her. He applauds Gatsby and declares Gatsby's great value, in comparison to the value of other characters in the novel.
If a painter makes more value judgments and does more interpreting than a photographer, then Nick is more of a painter than a photographer.