Art Institute of Chicago
Barnes Foundation (good filtering engine)
Europeana, 4.5 MILLION
Getty Museum, 150,000+
Met Museum, 492,000+ Public domain artwork pieces.
National Gallery of Art: 50,000+ public domain artwork pieces
New York Public Library, 940,000 scanned images
Paris Musees, 300,000+
Rijksmuseum, 350,000+
Smithsonian, almost 5 Million artwork pieces
Yale University Art Gallery, 180,000+