The Metropolitan Museum of Art has made 375,000 high resolution images from its collections free to use, with no restrictions. The images are in the public domain with a Creative Commons Zero license. You can search the images by Artist or Culture, Object Type or Material, Geographic location, Date or Era, or Department.
The museum has helpfully created 20 sets of themed images to start you on your way:

• Masterpiece Paintings
• Cats
• Monsters and Mythological Creatures
• Met-staches
• New York City
• Impressionism and Post-Impressionism
• Winter Wonderland
• Vincent van Gogh
• The Pre-Raphaelite Style
• Self-Portrait
• Quilts
• Gold
• Georges Seurat
• Arms and Armor
• The Monuments Men at The Met
• Faces from the Ancient World
• Tiffany Glass
• Dress to Impress
• Art or Design?
• Dishes

To identify free images, look for the Creative Commons Zero (CC0) icon directly below the image or use the Public Domain checkbox on the left side of the page. More information on the collection and how to use it is here:
Open Access Policy

You can see and search the collection here:
Metropolitan Museum of Art - Images of Artworks in the Public Domain

(via Crave)

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