From the Above us page:

In the last few years, several major museums and libraries have instituted an open access policy by designating most or all of the public domain art in their collections with a creative commons license making them available for use for any purpose with no restrictions attached.

We sort through and aggregate the best of these images in one location to make them easy to discover and download. […]

From the Terms of Service page:

Almost all of the downloadable images on the site are sourced from institutions around the globe that have adopted an open access policy to the public domain images in their collections, making them available to be used freely for any purpose, including commercial. […]