Laura,
According to the LoC ,that is Brigadier General Benjamin McCulloch, former Texas Ranger, First Lieutenant in the Texas Army, Major General of Texas Militia, and commander of the Missouri State Guard. Killed at Benton County, Arkansas, 7 March 1862.
He looks familiar, but the picture I am thinking of is a picture of him in old age. Its not Van Dorn, as Van Dorn has a "high forehead". And I think this guy is too young to be McCulloch, but didn't McCulloch have a little brother?
The picture is also credited to be Robert A. McCulloch of the 2nd Missouri Cavalry (hence the confusion with Ben McCulloch).
EDIT:
Family ties to Ben? Don´t know. But the Colonel was called Black Bob, and his Lieutenant Colonel was his cousin, who was also named Robert McCulloch and was called Red Bob.
Benjamin McCullough was born in 1811 (so 50 yo in 1861) and his brother Henry E McCullough was born in 1816 (so 45 yo in 1861) Seemingly they both would have been too old to be this guy? Van Dorn was born in 1820 so would have been 41 yo in 1861. Still seems a little older than the guy in the picture?
Benjamin McCullough was born in 1811 (so 50 yo in 1861) and his brother Henry E McCullough was born in 1816 (so 45 yo in 1861) Seemingly they both would have been too old to be this guy?
They are identical (with some enhancing) but that doesn't make the ID accurate. Based on McCulloch's other pictures, it's pretty clear that this is not him.