Last August the Getty Museum announced that it has made 4,600 pieces of art from the museum’s collection free to use. They’re focussing on “public domain” works of art, that means works that have endured beyond the limits of copy right, and users are free to use, modify, and publish these works for any purpose.
These are high-resolution, reproduction-quality images with embedded metadata, some over 100 megabytes in size. You can browse the images, or look for individual “download” links on the Getty Museum’s collection pages. Before the download actually begins, the Getty site asks simple questions about how you plan to use the images.
There’s a well-written Getty Open Content Program FAQ.
The Getty released images of many of its most famous works, including paintings like Vincent van Gogh’s Irises, but I’m especially excited by the medieval manuscripts (The Getty purchased the Ludwig collection, a huge collection of manuscripts rich in psalters and books of hours several years ago, and already had a solid collection, and they’ve added mss. since).