These two are not open-source, but are free to use. Yet they require you to use the company’s license in most cases; make sure to read the rights info!

• Getty Images is an enormous image library with hundreds of millions of media items.
• Pexels is a popular stock photo repository with more than 3.2 million media items.

• The National Institutes of General and Medical Sciences Image and Video Gallery has scientific photos, images and videos.
• The College of Physicians of Philadelphia Digital Library houses an image library of historical medical artifacts.
• The Cell Image Library offers visual media of (or related to) the cells of different organisms; not all images are license-free, so make sure to check.
• Bassett Collection of Stereoscopic Images of Human Anatomy offers a digital library images from a renown 1962 compendium of medical photographs and line drawings.

• The Metropolitan Museum of Art offers a searchable portal to tens of thousands of art images from its collection that the museum has released into the public domain.
• The Cleveland Museum of Art may offer a notably smaller collection than the Met, but it is still a large, easy-to-navigate and easy-to-search set of images from the museum’s holdings.
• The Smithsonian Institution collections include millions of open-access works; check out the Smithsonian’s statement on Open Access and using the 4 million+ visual assets in their collection.
• The site Digital Benin brings together images of art taken from Benin and dispersed across museum and private collections. Only a portion of the images are in the Public Domain, so be sure to check.

• Vintage photos from the public archives are available at the individually curated website New Old Stock.
• Browse the largest library in the world’s historic video, photographs, and more on the Library of Congress website.
• The David Rumsey Map Collection collects more than 150,000 maps and images from the 16th through 21st centuries; the works are licensed under a Creative Commons License that prohibits commercial use.
• To date, New York Public Library Digital Collections has 924,903 items—over 180,000 of which are in the public domain and reproducible as high-resolution digital downloads. Check out the NYPL’s page about using and sharing its public domain collections.

Google Image Search offers a quick way to sort for only images that are Creative-Commons licensed. To do so, do a regular Google image search:

Make sure you’ve selected “Image” and not “All”.

Note that the top row is always going to be ads, so be sure to scroll down after you change the kinds of images to Creative Commons licensed.

Under the Google search bar on the right is a button marked “Tools”. Click it and a submenu will appear, like this:

Choose the drop down arrow under Usage Rights.

Select Creative Commons licenses and then you’ll scroll down past the ads to see a new set of search results.

You will have to do a bit of extra leg work to find exactly what the CC license is for a particular image by clicking on the source listed below each image result.