The only known image believed to be from September 17, 1862, the day of the battle at Antietam.
This clearly shows the undulating terrain common throughout the battlefield.
See below for an enlarged view of the troops at the far right of this photograph.
Courtesy of the Library of Congress
Though familiar, the first image above draws the attention to the man in the foreground and the artillery to the left of the photo.
To the right, if viewed at a sufficient size, one can easily see several lines of soldiers, many of whom appear to be seated.
Courtesy of the US Library of Congress