Nick is always noticing pictures and photographs in people's homes. He calls one an ectoplasm (a ghost, spirit). So, there is a connotation of death connected to photos, supporting the novel's theme focusing on the "past." Nick's style of writing as mainly an observer narrator is very photographic in terms of point of view. He enters a scene and frames his subjects the way a photographer or might, according to rules of spatial organization. He maintains distance and objectivity this way (so he thinks) by using long shots (see below).
Also, the novel is a kind of memory photomontage: several negative images of subjects combined into one. The novel is told in flashback, from memory as Nick sits in the Midwest. Gatsby is framed entirely from flashback, all from memory. He is the centerpiece of the montage, with Daisy as its contrasting element.
“What was that?” I inquired.
“The picture of Oxford?”
Re: Mrs. McKee:
I gathered later that he was a photographer and had made the dim enlargement of Mrs. Wilson’s mother which hovered like an ectoplasm on the wall. His wife was shrill, languid, handsome, and horrible. She told me with pride that her husband had photographed her a hundred and twenty-seven times since they had been married.
Mr. McKee was asleep on a chair with his fists clenched in his lap, like a photograph of a man of action.
The living-room was crowded to the doors with a set of tapestried furniture entirely too large for it, ...The only picture was an over-enlarged photograph, apparently a hen sitting on a blurred rock. Looked at from a distance, however, the hen resolved itself into a bonnet, and the countenance of a stout old lady beamed down into the room.
In Chapter 1 Nick describes the Buchanans thusly:
The only completely stationary object in the room was an enormous couch...I must have stood for a few moments listening to the whip and snap of the curtains and the groan of a picture on the wall.
Chapter 3 begins this way:
There was music from my neighbor’s house through the summer nights. In his blue gardens men and girls came and went like moths among the whisperings and the champagne and the stars. At high tide in the afternoon I watched his guests diving from the tower of his raft, or taking the sun on the hot sand of his beach while his two motor-boats slit the waters of the Sound, drawing aquaplanes over cataracts of foam.
There are a couple of things I can think of.
First of all, photos we see from that era are generally black and white, but Fitzgerald uses much color as symbol. During that era people were physically drawing in color with pencils.
In the first couple chapters, Nick accompanies Tom to Tom's New York apartment wherein they pick up Myrtle, and Mr. and Mrs. McKee are there in NY. the only significance there is that Mr. McKee is a working class photographer. They discuss his success somewhat and things aren't overly successful.