Flickr CommonsFlickr Commons contains collections of images made available by museums and other organizations worldwide, including many images created by U.S. Government employees. These organizations have determined that "no known copyright restrictions" exist, which is not the same legal status as a license for unlimited reuse.
Each of the organizations that have contributed images to 'The Commons' has a rights statement that explains their understanding of "no known copyright restrictions" - read it before using an image and credit the source [https://www.flickr.com/commons/institutions/]. Images provided by the United States Federal Government are free of copyright and available for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations, and Web pages.