Man Ray. Observatory Time - The Lovers. 1936.
The question of what qualities a photo must possess to be considered "art" is both difficult to answer and subject to considerable debate- in short there are no definitive criteria. With that being said however, there are some general things to look for; clues that tell us that art was the photographer's intent (whether we agree that it was accomplished or not).
I think an important point is made in the text with the following assessment: "We should distinguish between claims made by photographers for themselves, or for particular movements, and claims made at a later stage by historians, critics, and curators" (PCI 262). In other words, the photo (and all art) has to stand up to the test of time. If Man Ray had stated in 1936 that the above photo was art- it wouldn't necessarily have made it so. But here, many decades later, most critics and art historians would agree that it qualifies. We're all aware of how the value of art goes up once the artist that created it is dead. True recognition and status as an artist takes time.
The techniques vary for photography as art, but one common method is how the photo is staged. The surrealists, of which Man Ray was a member, were not photographing natural landscapes or subjects as in straight photography. They were looking for unusual subjects and props that were more like elements of a dream than reality. The nude and props in the above are not an ordinary scene we'd encounter in waking life.
But there were also techniques that manipulated the medium more directly, such as another work by Man Ray at right. A camera was not even involved; this is a photogram (he called them "rayographs") which is made by placing objects on photographic paper and then exposing it to light. Again, this isn't an attempt to represent an image realistically; it's much more of an abstract "painting" that's been created photographically.
Art photographs share many aspects with other forms of photography. Considerations such as perspective, focus, composition, or cropping are common to most photos. I think the difference is that art photos attempt to convey more of a hidden message; a statement that goes considerably beyond the subjects and the photos themselves.
Second photo: Man Ray. Ray-o-graph III. 1927.