The attribution of this photograph to Chickamauga appears to come from
a 2007 listing at Heritage Auctions. Here is the description:
1/4th Plate Tintype Image of a Large Group of Union Soldiers Taken Before the Battle at Chickamauga of about 50 armed Union soldiers in various poses which the consignor indicates was taken shortly before the battle at Chickamauga, Georgia in September of 1863. The clear image provides a wonderful study of uniforms and equipment. Bayonets are fixed and the faces of the men appear tense. One is struck by the youth of the men in this unidentified regiment. The image is housed in half of a case and still retains the gilt brass frame. There is a surface crack in the lower third of the image, else it is sharp, clear and in very good condition.
So, the person selling the image says it's from Chickmauga, but no documentation is offered. Okey-dokey.