The official Library of Congress subject heading for World War I is "World War, 1914-1918." Many, but by no means all, of the World War I photos in the collections have this subject heading. To find other photos, you can search for terms used at the time for the war, including those used by other countries. Examples include World War, Great War, European War, Le Grande Guerre, and Weltkrieg.
The Prints & Photographs Division has multiple collections of photos taken or distributed by American, British (and its dominions), French, and German sources. The division has smaller numbers of photos from Austro-Hungarian, Belgian, Serbian, and Japanese sources. Photos in and around Palestine can be found in the Matson Collection, which contains images taken during both Ottoman and British rule.
The Harris & Ewing Collection, National Photo Company Collection, and Bain Collection have large numbers of digitized photos. The subject matter tends to be based around Washington, DC, and New York City. They include photos of soldiers training, parades, visiting foreign military and diplomatic groups, and fund raising activities. Bain would sometimes copy photos from other sources. As a result, this collection includes some images taken in Europe showing soldiers of other combatant countries. Since Harris & Ewing and National Photo Company were based in DC, they have strong coverage of politicians, diplomats, and such events as the dedication of the Tomb of the Unknown Soldier. The Harris & Ewing Collection is a good place to start when you are looking for preparedness activity before the war or photos on the home front during the war.
The Panoramic Photograph Collection is digitized and has images of US training camps, bases, groups of soldiers, and post-war landscapes of battlefields and battle damaged towns. The panoramic photo format works well for showing large groups of people and a wide viewing range. The Prints & Photographs Division does not have photos of every group of soldiers who went to a training camp during the war. Most of the panoramas were acquired through copyright deposit.
Stereograph cards of World War I include a diverse array of images and are a good starting point when looking for scenes from this war. The World War I stereographs are primarily in two places. The first is LOT 14008 which is a 300-card set of stereographs produced by the Keystone View Company. The set shows various facets of the war, although the majority of the photos were taken on the Allied side. The back of each card has an extensive caption. The set includes a book entitled, "The World War Through the Stereoscope; A Visualized, Vitalized History of the Greatest Conflict of All the Ages," which has been digitized. All of the cards in this set have been scanned, front and back, and can be found in the online catalog.
The second large group of World War I stereograph cards is in LOT 11527. It has over 1,000 cards arranged according to the company that published the cards. All of the stereographs in this LOT have been scanned.
Over 18,000 negatives from the American National Red Cross collection are digitized and available in the online catalog. The majority of these photos were taken during World War I and show activities of the American Red Cross in the United States and overseas. It also includes photos of refugees, orphans, and other people in Europe who were affected by the war as well as soldiers from many of the combatant countries. Some of the most requested images from the collection relate to masks made by Anna Coleman Ladd for French soldiers with facial injuries. To limit search results to photos from the American National Red Cross Collection add "anrc" to your search words. For example, if searching for nurses in the collection, type in the search box: anrc nurse. The photographic prints in this collection are currently being processed.