The men in this photo (taken abt. 1912) I believe (but can't prove except for J.M. O'Neal) to be:
L-R. Ambrose Porter (1839-1916), James Gaffney (1834-1923), Capt. George Steck (d. 1921), and Joseph Milton O'Neal (1843-1917)

Ambrose Porter. Geo. Steck, James Gaffney, J.M. O'Neal (and others) were all present (or swam) during a swim across the river to rescue a skiff of supplies tied on the enemies bank (during the Civil War) that the Union army needed. This story is online at findagrave.com under Ambrose Porter (d. Holt Co., MO)
  "His service in that regiment led to his being mustered in as Commissary Sergeant of Company D, 12th Missouri Volunteer Infantry. It was while he was with this unit that he performed an act of bravery during an engagement on the Tallahatchie River, Mississippi on August 7, 1864 that would see him awarded the CMOH. The citation for the award reads "Was 1 of 4 volunteers who swam the river under a brisk fire of the enemy's sharpshooters and brought over a ferry boat by means of which the troops crossed and dislodged the enemy from a strong position." He was awarded his Medal of Honor for this on August 24, 1905. He was later promoted to 1st Lieutenant before being mustered out of the 12th Infantry."
   Those from Co. D that were captured on the 24 Nov 1864 at Campbellville, TN (from Atchison) were Geo. Steck, H. Rogers, O? S., F. Pucket?, Jos. O'Neal, Lambert Johnson, James Gaffney, George Mann, William Pugh, George Thorpe. J. M. O'Neal ended up at Andersonville prison.
  My hope is that others that have photos might be able to positively identify these first three men..or even possibly the history center might be able to shed some more light on them.
 
Photo and data submitted by: Kathy Urban - kwug54@gmail.com