I like the first answer for as far as it goes (especially in reference to the portfolio and the titles of the photographs). However, I think it's also important to remember that Nick is writing a novel about his experiences, so his use of ellipses is his novelistic technique, not just Fitzgerald's. Nick might have used the ellipses to indicate his drunken state, but if that's the reason, why didn't he use ellipses earlier? Nick was drunk earlier in the evening, too, and yet, this is the first place where he uses ellipses.
Some critics suggest that Nick uses ellipses to indicate a possible intimate homosexual encounter between Nick and Mr. McKee, who he earlier described as a "pale, feminine" man who is uninterested in his wife. Also, it might be helpful to remember that Nick is obsessed with the spot of dried lather on Mr. McKee's cheek and eventually takes out his handkerchief to wipe it off when Mr. McKee fell asleep on a chair. Mr. McKee invites Nick to lunch when they are taking the elevator from Myrtle's apartment. I think drunkenness does not fully explain these last lines, in which Nick specifically explains that Mr. McKee is "sitting up between the sheets, clad in his underwear." Mr. McKee's near nakedness and the two men's location in the bedroom suggests something quite intimate has occurred. Also, Nick's use of ellipses to leave out details suggests he may be trying to avoid something: his relationship with Mr. McKee, the events of the evening with Mr. McKee after he leaves Myrtle's apartment, or perhaps just his own drunken behavior.
This is a great passage! Thanks for asking such a good question.