I came across this information in a PDF, it's a list of US Gov websites with royalty free photos.
I thought someone might find it useful.

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Digital Imaging Information (University at Albany)

Royalty Free Photos
From the US Government

The United State Government is millions of photographs available for use by the general public. These photos are generally royalty free. Many units of the government have made collections of photographs available online. The list below is a selection of US Government Websites of particular interest. Other photograph collections can be found by exploring other Federal sites.

Be sure to check the permissions for terms of use. All photos by the Government should be freely available for use, but there may be exceptions or limitations.

FirstGov Official US Government Portal – Main list of Federal photo collections.
http://www.firstgov.gov/Topics/Graphics.shtml

US Department of Agriculture – Wide variety of photographs ranging from food and farming to 4-H clubs and animals.
http://www.ars.usda.gov/is/graphics/photos/

NASA Images – Collection of images associated with space and the space program. Includes photographs of missions and links to multimedia files.
http://www.nasa.gov/multimedia/imagegallery/index.html

NOAA Photo Library – Ten of thousands of photographs covering just about any topic associated with weather, the oceans, explorations, and even space.
http://www.photolib.noaa.gov/index.html

National Park Service Digital Image Archives – This is a gigantic searchable database that returns images based on location or subject. Stunning photographs from the national parks and other sites administered by the Service.
http://www.nps.gov/pub_aff/imagebase.html

Smithsonian Institution – A wide range of photographs that includes everything from dinosaurs to fireworks. Some "how-to" photo articles are available.
http://siarchives.si.edu/collections