The Getty's Open Content Program

Tens of thousands of downloadable, hi-res images from the Getty Museum collections and the Getty Research Institute collections including antiquities, illuminated manuscripts (!), paintings, sculpture, photographs, maps, prints and study images.
Handy FAQ here.

🖼️ Image info: An Allegory of Painting, 1661, Frans van Mieris the Elder. Oil on copper, 5 × 3 1/2 in.