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).
Jean-Claude-Richard, Abbé de Saint-Non. Fragments choisis dans les Peintures et les tableaux les plus intéressants des palais et des églises de l'Italie (Fragments chosen from the most interesting paintings and canvases of the palaces and churches of Italy), 1767-1775. National Gallery of Art, Washington.