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.
Digital Imaging Information
(University at Albany)
Royalty Free Photos
From the US Government
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