Government or Library Archives
The U.S. Library of Congress (LOC) has enormous collections of photographs and other digitized holdings, including military subjects. Only a fraction of the images are on line, but the number is increasing all the time. The American Memory page allows you to browse their collections. The Prints & Photographs Reading Room has links to dozens of other points of entry to the LOC's vast holdings, including the Prints & Photographs Online Catalog. The LOC also offers this guide to Sources of U.S. Military Images. Unfortunately most are not on-line downloads, but this list of locations and contact information tells you where to find many types of images from U.S. military history.
US Army Signal Corps Hampton Roads VA Port of Embarcation Collection. This is from WW II and has some beautiful B&W photos including the one of the WAC in the jeep on the first page of the Military Vehicle Ownership Guide.
The photographs of the Farm Security Administration-Office of War Information Collection include scenes of rural and small-town life, migrant labor, and the effects of the Great Depression. A significant number of the photographs, both color and black/white concern the mobilization effort for World War II and portray aircraft manufacturing, military training, and the nation's railroads. The 1,600 color photographs produced by the FSA and OWI photographers are less well known and far less extensive than the 164,000 black-and-white photographs in the collection. Use their search engine to find photos by key words such as "Fort Knox". (Thanks to David Robinson, of Brisbane, Australia for this suggestion.)
The Franklin D. Roosevelt Library, Museum, and Digital Archives is an on-line resource for scholars, teachers, students and members of the general public to gain access to a portion of the rich collection of documents, photographs, sound and video recordings, finding aids, and other primary source materials found at the Franklin D. Roosevelt Library in Hyde Park, New York.
National Archives and Records Administration Archival Research Catalog (ARC) contains more than 348,000 descriptions and 60,000 digital copies, representing only a limited portion of NARA's vast holdings. This is the on-line, searchable catalog of the National Archives which will be updated until all records are available. Click on the photo (right) for a larger image and the caption. Click on this link for more on their collection: "Pictures of World War II". or on this link for "Pictures of African Americans During World War II".
The National Museum of Health and Medicine has a photo gallery titled Battlefield Surgery 101: From the Civil War to Vietnam.
Smithsonian Images: Digital Photography, Printed Pictures, Historic Images, and E-Cards from the Office of Imaging and Photographic Services of the Smithsonian Institution.
This collection contains digitized photos of many of the unique research aircraft flown at what is now known as NASA Dryden Flight Research Center at Edwards, California. These images date from the 1940s to the present.
The New York Public Library Digital Gallery provides access to over 275,000 images digitized from primary sources and printed rarities in the collections of The New York Public Library, including illuminated manuscripts, historical maps, vintage posters, rare prints and photographs, illustrated books, printed ephemera, and more. You can search for photos using military key words.
The Western History / Genealogy Department of the Denver Public Library has a huge collection of photographs including many with military subject matter. The series on the 10th Mountain Division is of particualr interest regarding World War II.
The library at Brigham Young University hosts the full text of Scott's Official History of the American Negro in the World War, a comprehensive study of African American participation in World War I. In addition to the text, they have made available for download a long list of photo illustrations from the book.
The University of West Alabama, Julia Tutwiler Library, Alabama Room Special Collections includes this set of Official World War Two Photographs from all branches of the services.
Virginia Fights World War II is an image archive from a documentary film. Has over 1,600 photographs from the personal collections of the individuals interviewed for the film plus others from the National Archives, the Library of Virginia, and other institutions holding World War II Virginia images. Many duplicates of other archives but well worth a visit.
World War II: Through the Eyes of the Cape Fear is a joint project between the University of North Carolina's William M. Randall Library and the Cape Fear Museum. A valuable collection of unique photographs, artifacts, and recordings including photos documenting World War II uniforms, clothing, weapons and equipment.
Find More Information on the Web
There are many fine websites that have additional information on this topic, too many to list here and too many to keep up with as they come and go. Use this Google web search form to get an up to date report of what's out there.
For good results, try entering this: military photos. Then click the Search button.