R.D., you are probably correct in assuming that F. D’Avignon was the artist. From an inquiry on rootsweb I found that Francis D’Avignon was an artist/lithographer. He was born in France about 1813. He enlisted on 25 May 1861 in Company I, 2nd Massachusetts Infantry, and fell captive during the first Battle of Winchester, VA, 25 May 1862, but was back with his unit by July 1862. It is possible that Francis drew the soldier’s picture while in captivity. Unfortunately, Fold 3, does not have his unit, so I could not verify this information.

The inquiry contains more information about his service, and it can be found at:
http://archiver.rootsweb.ancestry.com/th/read/CIVIL-WAR/2007-12/1199120285