Colecciones digitales
Wikimedia Commons
A volunteer-maintained media library with public-domain and openly licensed artworks; check each file's license, and access from mainland China is usually blocked.
GratisInternacionalMultilingüeLicencia abierta
Who it is for
People who need reusable images and will check each file's license.
How to use it
Search for an artwork or artist, then check the license and source on the file page.