Was he onboard a ship in the Navy? Include a photo of the ship from the U.S. Naval Historical Center's Photographic Unit.
(http://www.history.navy.mil/photos/sh-usn/usn-name.htm)
What was the insignia for the military unit he served in? See the section below on insignias, medals, etc. Be creative. Was your great-great-grandfather one of the soldiers who listened to Abraham Lincoln's Gettysburg Address? If so, a copy of the address (http://lcweb.loc.gov/exhibits/gadd/) can bring an interesting element to the story, indelibly linking him to the history of the country.

Historical and Miscellaneous Information
U.S. Army Military History Institute--Historic Documents
http://carlisle-www.army.mil/usamhi/DL/
Historical documents can add an extra element to our family history. It places our veteran ancestors in the context of history. Browse a huge selection of documents at:
http://carlisle-www.army.mil/usamhi/DL/AtoZ.htm

U.S. Army Center of Military History
Location:
Building 35
102 Fourth Avenue
Fort McNair
Washington, DC
Mail:
103 Third Avenue
Fort McNair, DC 20319-5058
Tel: 202-761-5413
E-mail: cmhweb@cmh-smtp.army.mil
http://www.army.mil/cmh-pg/default.htm
This site often features revealing articles and online exhibits. Check back often as this page is updated regularly. Also check out the Army Division Matrix, which includes insignia information, lineage and honors, photos, unit decorations, sound and video, division commanders, order of battle, bibliography, interviews, and war service by division. Current online exhibits include: "Remembering the Korean War A Prelude to the Official Commemoration;" "The German Campaigns in the Balkans;" "German Antiguerrilla Operations in the Balkans (1941-1944);" and "Images of West Point."

Naval Historical Center
Washington Navy Yard
901 M Street, SE
Washington, DC 20374-5060
Library:
Tel: 202-433-4132
Fax: 202-433-9553
Museum:
Tel: 202-433-4882
Fax: 202-433-8200
Operational Archives:
Fax: 202-433-2833
Ships History Branch:
Tel: 202-433-3643
Fax: 202-433-6677
http://www.history.navy.mil/
Loads of information here on U.S. Naval history. Sections include: "War & Conflicts of the U.S. Navy;" "Traditions of the Naval Service;" "Ships History;" a photographic section; information on facilities; bibliographies; and much more.

Photos
U.S. Army Military History Institute
22 Ashburn Drive, Carlisle Barracks
Carlisle, PA 17013
Tel: 717-245-3611
Email: MHI-SC@carlisle-emh2.army.mil(Special Collections)
MHI-AR@carlisle-emh2.army.mil (Archives Collection)
MHI-HR@carlisle-emh2.army.mil (Historical Reference)
http://carlisle-www.army.mil/usamhi/
Historic Photographs Online
http://carlisle-www.army.mil/usamhi/HPOL.html
Photographic Database
http://carlisle-www.army.mil/usamhi/PhotoDB.html

Naval Historical Center--Photographic Section
http://www.history.navy.mil/branches/org11-2.htm

National Archives' NAIL database
http://www.nara.gov/nara/nail.html
Search NARA's digital collection for photographs. I tried a search for "machine gun battalion" because my grand uncle served in the 105th in World War I. The search produced eight hits, one of which was a group photo of the 129th Machine gun Battalion. Although I did not locate a photo of my grand uncle's battalion, I did locate a photo of "American troops going forward to the battle line in the Forest of Argonne, France, September 26, 1918" and I know from previous research that he was in the area around this time.

Civil War Photographs at the Library of Congress (American Memory Collection)
http://memory.loc.gov/ammem/cwphome.html

Insignias, Medals, Ect.
U.S. Army Institute of Heraldry
9235 Gunston Road, Room S-112
Fort Belvoir, VA 22060-5579
Tel: 703-806-4968
703-806-4969
http://www-perscom.army.mil/tagd/tioh/tioh.htm

Military Insignia Information Center
http://www.naples.net/clubs/zmilins.htm
Also has a great collection of military links at:
http://www.naples.net/clubs/milsites.htm

Americal Division Veterans' Administration: Army Awards and Decorations
http://www3.servtech.com/americal/awards/

Full-text Listings of Medal of Honor Recipients
http://www.army.mil/cmh-pg/moh1.htm

U.S. Army Center for Military History--Lineage and Honors Certificates
http://www.army.mil/cmh-pg/lineage/lindex.htm

"Armies, Corps, Divisions, and Separate Brigades" by John B. Wilson
http://www.army.mil/cmh-pg/BOOKS/Lineage/ACDSB/ACDSB-notice.htm
CMH publication for sale--page has links to selected images online.

Veterans Day Pages
Department of Veterans Affairs Veterans Day Page
http://www.va.gov/pubaff/vetsday/index.htm

Remembering Our Veterans and Selfless Service to Our Country
http://www.army.mil/cmh-pg/faq/vetsday/vets1.htm