USGS Denver Library Photographic Collection
This is a collection of >500,000 photographs, slides, glass plates, lantern slides, negatives, and sketches from the 1870s onward taken by USGS scientists during field studies. Subjects encompass a broad variety of earth science disciplines. The web site provides free downloads. USGS-authored photographs are in the US Public Domain. However, you will need to check the author credit information on the images — some of them are non-US Government and might require contacting the copyright holder to use.