Creative Commons
"You are free to:
Share - copy and redistribute the material in any medium or format for any purpose, even commercially.
Adapt - remix, transform, and build upon the material for any purpose, even commercially.
The licensor cannot revoke these freedoms as long as you follow the license terms." - Creative Commons Attribution 4.0 International license.
Library of Congress
"The Library of Congress is the largest library in the world, with millions of books, films and video, audio recordings, photographs, newspapers, maps and manuscripts in its collections. The Library is the main research arm of the U.S. Congress and the home of the U.S. Copyright Office."
- About the Library
NASA Archives
"NASA content - images, audio, video, and media files used in the rendition of 3-dimensional models, such as texture maps and polygon data in any format - generally are not subject to copyright in the United States. You may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages. This general permission extends to personal Web pages."
- NASA Images and Media Usage Guidelines
Smithsonian Open Access
"you can download, share, and reuse millions of the Smithsonian's images - right now, without asking. With new platforms and tools, you have easier access to more than 5.1 million 2D and 3D digital items from our collections - with many more to come. This includes images and data from across the Smithsonian's 21 museums, nine research centers, libraries, archives, and the National Zoo."
Wikimedia Commons
"Wikimedia Commons is a media file repository making available public domain and freely licensed educational media content (images, sound and video clips) to everyone, in their own language. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted here."
- What is Wikimedia Commons?