I don't think I agree with this conclusion. I don't think it's coincidence, but I do think that it was probably the best shot that particular photographer had of Patreaus with any of the candidates.
I really think your reading way too much into the photo. The other option of course, was to get a photo of Obama, Clinton, and McCain together, thus representing a "balanced" photo.
Of course it's not going to happen. So if Patreaus was shot with McCain would it be propaganda? No. What about just Obama? Or Obama and Clinton? No and no. Really, it's a "no win" proposition here given the choices the editors had. Either go with Obama, Clinton or McCain--I mean, when is a photograph just a photograph and not "propaganda?"
The story is about the future presidents' interaction with Patreaus. This shot was likely the best one. I wouldn't read any more or less into it.
I think the most logical reason for the photograph is probably simple--either the photographer didn't get a shot of Patreaus with any of the other candidates or this particular one was the best. Really, I think we can go overboard here in picking apart every press story, photograph, etc.