From their website:
"The US Army Corps of Engineers' (USACE) Digital Visual Library is an evolving collection of visual products covering every major USACE program, project, and activity. The library consists of photographs, illustrations, artwork, clipart, logos/registered trademarks, and maps. The images are submitted by all USACE commands and are available, free, to all internal and external public audiences. This is the Corps' main source for publication quality images and graphics. To further assist you, contact information is provided with each image.
The items contained in this library may be downloaded for free, however, where appropriate please give credit to the US Army Corps of Engineers".