OFFLINE

Most are public domain

Most public domain photos were created by U.S. Government employees as part of their official duty. These photos are not copyrighted, so they may generally be used freely without restriction provided there are not underlying legal rights that may limit your use of a photo. Privacy rights and trademark laws are just two examples of such possible limitations.( This means avoid using photos with Logos or identifiable people.)

JSC Digital Image Collection
http://images.jsc.nasa.gov/luceneweb/browse.jsp
NASA Johnson Space Center Facility

NASA Human Space Flight Gallery
http://spaceflight.nasa.gov/gallery/index.html
If the NASA material is to be used for commercial purposes, especially including advertisements, it must not explicitly or implicitly convey NASA's endorsement of commercial goods or services.

NOAA Photo Library
http://www.photolib.noaa.gov/index.html
The NOAA Photo Library has been built so as to capture the work, observations, and studies that are carried on by the scientists, engineers, commissioned officers, and administrative personnel that make up this complex and scientifically diverse agency.

U.S. Geological Survey Photographic Library
http://libraryphoto.cr.usgs.gov/
The U.S. Geological Survey Denver Library maintains a collection of over 400,000 photographs taken during geologic studies of the United States and its territories from 1868 to the present.

These images provide a visual history of the discovery, development, and sciences of the United States and its Geological Survey. Some photographs have been used in USGS publications, but most have never been published.

Currently, this website represents less than 10 percent of the Library's images with approximately 30,000 photographs on-line.

BestPhotos.US
http://www.bestphotos.us/
Landscapes Animals Plants
Black & White Food Tech & Econ
Recreation

ARS Image Gallery - Agricultural Research Service
http://www.ars.usda.gov/is/graphics/photos/
The Image Gallery is provided as a complimentary source of high quality digital photographs available from the Agricultural Research Service Information Staff.
Beautiful, high-resolution photos of plants, animals, agriculture, and agricultural research.

U.S. Fish and Wildlife Service National Digital Library
http://digitalmedia.fws.gov/
I love this site
Photos of scenery, wildlife, people in the outdoors

U.S. Department of Defense
http://www.defense.gov/multimedia/multimedia.aspx
Photos of soldiers, vehicles, and operations in Army, Air Force, Navy, and Marines.

U.S. National Park Service Photo Galleries
http://www.nps.gov/photosmultimedia/photogalleries.htm
Astounding amount of photos
Did you know almost all parks run by NPS have flickr photostreams
Example:
North Cascades National Park's photostream

U.S. Government Photos and Images: USA.gov
http://www.usa.gov/Topics/Graphics.shtml
USA.gov: U.S. Government Photos and Images -- Links to public domain graphics, photos and images that may be used and reproduced without permission or fee.

Large list of GOV sites