The Library of Congress maintains a list of major repositories of U.S. military images, which includes many online resources that provide old U.S. Army photographs. Wikimedia Commons also hosts photographs related to Army operations and personnel.
U.S. military photography, including materials showing U.S. Army involvement in past conflicts such as World War II, is available from the National Archives website.
Wikimedia Commons hosts a variety of old military photographs and public domain content. This ranges from photos of personnel, such as General Seth Williams during the U.S. Civil War, to war plans, such as maps showing locations of American-Indian battles.