License, verbatimPD
The National Gallery of Art: open access image with no usage restrictions under the NGA's open access policy (openaccess=1 in the Gallery's published_images dataset).
Eadweard Muybridge. The Horse in Motion as Shown by Instantaneous Photography. With a Study on Animal Mechanics Founded On Anatomy and the Revelations Of The Camera. Executed and Published Under The Auspices of Leland Stanford, 1881-1882. National Gallery of Art, Washington.