The Getty Museum has over 114,000 images you can use for free. Most are historical works of art, including many types of images such as paintings, prints, jewelry, manuscripts, photographs, architecture, maps, playing cards, sculpture, decorative arts, artists' sketchbooks, and much more.
The images are high quality and more are added regularly to the collection.
Images are offered through the Getty Museum's Open Content Program and accessible through the Getty Search Gateway, where you can search the collection by type (photograph, manuscripts, jewelry, figures, architecture, map, etc.), by name, by place (France, Scotland, Japan, Egypt, etc.), and by source.
Images are free to use with proper credit given to the Open Content Program (details here).
You can find more Tech Treats here.