Art Institute of Chicago Collection

One of the friendliest museum websites to use, and an early mover on CC0: since 2018 the Art Institute has released images of its public domain works for any use, launching with more than 44,000 and promising to add more. Pages are clean, the images are generous, and it is easy to go from a single object to related works by the same artist or period. The catch is the one every open access collection shares, and here it bites hard: many of the museum's most famous pictures are modern and still in copyright, so Nighthawks and Picasso's The Old Guitarist are outside the free image policy. For Impressionism and earlier, it is a joy.