National Gallery of Art: Public Domain Images 

https://images.nga.gov/en/page/show_home_page.html

NGA Images is a repository of digital images of the collections of the National Gallery of Art. On this website you can search, browse, share, and download images. A standards-based reproduction guide and a help section provide advice for both novices and experts. More than 45,000 open access digital images up to 4000 pixels each are available free of charge for download and use (mostly public domain). NGA Images is designed to facilitate learning, enrichment, enjoyment, and exploration.

Unsplash: High Resolution Photos

https://unsplash.com

All photos published on Unsplash are licensed under Creative Commons Zero which means you can copy, modify, distribute and use the photos for free, including commercial purposes, without asking permission from or providing attribution to the photographer or Unsplash.

Do whatever you want free high-resolution photos. Download 10 new free photos every 10 days.

Source: License | Unsplash

Wikimedia from Wikipedia

https://commons.wikimedia.org/wiki/Main_Page

Almost all of the multimedia content hosted on Wikipedia may be freely reused subject to certain restrictions. You do not need to obtain a specific statement of permission for using the content unless you wish to use the work under different terms than the license states.

Content under open content licenses may be reused without any need to contact the creator, but just keep in mind that:

  • some licenses require that the original creator be attributed;
  • some licenses require that the specific license be identified when reusing (including, in some cases, stating or linking to the terms of the license);
  • some licenses require that if you modify the work, your modifications must also be similarly freely licensed; and finally,
  • While the copyright and licensing information supplied for each image is believed to be accurate, the Wikimedia Foundation does not provide any warranty regarding the copyright status or correctness of licensing terms. If you decide to reuse files from the Commons, you should verify the copyright status of each image just as you would when obtaining images from other sources.

Wellcome Images Collection

http://wellcomeimages.org/

Over 100,000 images ranging from ancient medical manuscripts to etchings by artists such as Vincent Van Gogh and Francisco Goya are now available for free download as hi-res images on the Wellcome Images Collection website.

Drawn from the historical holdings of the world-renowned Wellcome Library, the images are being released under the Creative Commons-Attribution only (CC-BY) licence. This means that all the historical images can be downloaded here to freely copy, distribute, edit, manipulate, and build upon as you wish, for personal or commercial use as long as the source Wellcome Library is attributed.

The historical collections offer a rich body of historical images including manuscripts, paintings, etchings, early photography and advertisements. The earliest item is a 3000 year old Egyptian prescription on papyrus, and treasures include exquisite medieval illuminated manuscripts and anatomical drawings, ranging from delicate 16th century fugitive sheets, whose hinged paper flaps reveal hidden viscera, to Paolo Mascagni’s vibrantly coloured etching of an ‘exploded’ torso.

From the beauty of a Persian horoscope for the 15th-century prince Iskandar to sharply sketched satires by RowlandsonGillray and Cruikshank, the collection is sacred and profane by turns. Photography includes Eadweard Muybridge’s studies of motion, John Thomson’s remarkable nineteenth century portraits from his travels in China and a newly added series of photographs of hysteric and epileptic patients at the famous Salpêtrière Hospital.