In addition to P.F's response, you can try the Army website, which breaks down into many links that can assist you. Also try the Center for Military History : www.army.mil/cmh-pg/default.htm and the Military history institute at Carlisle, http://carlisle-www.army/usamhi/ which has an online photo base. For other branches, try the same thing, go to the branch website and look for links.

Specialized units, ships and services also had specialized yearbooks-so you might find unit photos-and also separate books for particular duties, example the USN that were assigned to VXE-6 in Antarctica under the National Science Foundation published separate "wintering over" volumes.