Susan Sontag, in her book, On Photography asserts that [paraphrasing] "all photographs become art once they reach a certain age." One can verify this by examining the photographic holdings of most any museum. It's a matter of simple annointment. The more influential the person executing the annointment, the wider the sphere of possible effect.
If there were a required reading list for visitors to this "Art" forum, it would possibly raise the level of discourse or reduce the periodicity of the familar cycle of predictable exchange. In that spirit, I would suggest the following for those interested:
Sontag, Susan, On Photography, Picador, New York, 1973, 1974, 1977
Danto, Arthur C., After the End of Art, Princeton University Press, NJ, 1997
Greenhalgh, Paul, The Modern Ideal, the Rise and Collapse of Idealism in the Visual Arts,V & A, London, 2005
Barrett, Terry, Criticizing Photographs, Fourth Edition, McGraw Hill, Boston, 2006
Adams, Robert, Beauty in Photography, Aperture, New York, 1996