Jim, William Frassanito identified them as most likely being members of Starke's brigade in his book "Antietam: Photographic Legacy of America's Bloodiest Day." The 8th Louisiana (Hays' Brigade) was positioned farther to the east and fought in the infamous cornfield. I have an article coming out in the next day or so in the New York Times "Disunion" series about that photograph and another one Alexander Gardner took of Col. Henry Strong's (6th La.) dead horse. Here's a link to the NYT series.

http://opinionator.blogs.nytimes.com/category/disunion/