Several art experts have examined the photograph and stated they believe it was created by the photography studio of Mathew Brady, a famous 19th-century photographer. The photo probably was not taken by Brady himself but by Timothy O'Sullivan, one of Brady's apprentices. O'Sullivan took a multitude of photos depicting the carnage of the Civil War.
You can read more and view the photograph at the KSL web site at http://www.ksl.com/?nid=157&sid=11119336
My thanks to Laura Moore for telling me about the article and photograph.