The Los Angeles County Museum of Art (LACMA) has a over 20,000 works from their collection online. They are available as public domain for high-res download and use for free. I stumbled upon this sometime last year and it has been my go-to sight since.
Just yesterday a colleague posted a link to an article in The Atlantic which includes a fabulous list of FREE IMAGE COLLECTIONS on the web! Perfect! I've made it easy and put the links below:
Getty Search Gateway
Rijks Studio The digital image collection of the Netherlands's state museum.
National Gallery of Art
Wikipedia Featured Pictures
Library of Congress Prints & Photographs Online Catalog
Washington State Coastal Atlas