Here is an excerpt from an excellent article by Harold Merklinger over at Luminous Landscape:

I would argue that an image, whether a painting, a drawing or a photograph, is a graphic portrayal of some object, scene, story, or idea that conveys some message to the viewer through the sense of sight. Even a drawing or painting may involve some degree of technical assistance in its preparation, be it a measure taken by means of a brush handle and thumb, a projected image, a sketch or even a photograph. It may also be entirely the imagination of the artist. A photograph necessarily relies heavily on technical assistance, but also involves some degree of human intervention. A human probably at least determined where to point the camera and when to push the button. A photograph necessarily requires some scene or object to serve as its origin. That origin does not need to be identifiable, but it usually helps to have at least a few clues. From there on, it depends what photographic game the photographer chooses to play. Is the scene identifiable? Is there some action or situation that is to be portrayed? Is it a play on colour and/or light? How much detail or lack thereof is appropriate? Am I satisfying just myself, or do I have a viewer or audience in mind? Does the viewer understand or need to understand my intent? The answers will vary.

You can read the rest of the article here