(written from a Production point of view)
Lewis R. Stegman (18 January 1839 – 7 October 1923; age 84) was an American Civil War soldier most notably known to fans of the Star Trek franchise for appearing as the photographic representation of Thaddius Riker, an ancestor to William T. Riker, who was seen in the Star Trek: Voyager episode "Death Wish".
The original photograph of Stegman, published on page 289 of F. Miller's The Photographic History of the Civil War, vol. 7 (1911), depicts him standing next to a Lieutenant Donner from Ohio, however, for the photograph's appearance in the episode, an image of Gerrit Graham as Quinn was imposed over that of Donner's face. 
The following was stated of Stegman on the original page the photo appeared:
A Federal Officer Wounded At Pine Mountain, Georgia – August, 1864
This unusual photograph of an officer still on crutches, emasculated and suffering, was taken in August, 1864, near Pulpit Rock, Lookout Mountain, Tennessee. It is reproduced here through the courtesy of the officer himself – Major (later Colonel) L. R. Stegman, associated with the editors in the preparation of this work. In June, 1864, during Sherman's march to Atlanta, he was shot in the thigh, the short fracturing the bone. Major Stegman was in command of the Hundred and Second New York, which was attached to the twentieth corps of the Army of Cumberland. A wound of this character disabled the victim for many months. Colonel Stegman's companion in the photograph is Lieutenant Donner, of an Ohio regiment, also wounded in the thigh and using a cane for support.