"Army pictures" as we know them today - officer and enlisted portraits that become part of a soldier's file - is a fairly modern phenomena. Most military ID cards issued during World War II did not have a photo, just a description. The photos you usually see were taken by private photographers, usually family members or by a professional photographer at their studio. For example, my parents engagement/wedding photograph (it amounted to the same thing) was taken by a photographer in New Kensington, PA. Typically, any military photos were taken as unit photos and closely resemble class photos. For example, Cannon Company, 162d Infantry:
First, you need to know what unit you are looking for and when; you are literally talking about thousands of photos. There is also no guarantee that the person you are looking for was included in the photo or that you will even recognize them (my Dad is virtually unrecognizable in his VPI Corps of Cadets photo). If you can narrow that down, the U.S. Army Military History Institute and Army History and Education Center at Carlisle Barracks holds a large repository of photos. They would be a good start for an inquiry.
I know a distant cousins unit I have been researching but have no idea on either of my grandpas. Thank you for the information. Will look them up