There are more than 30 million freely usable media (image and/or audio files) in this database. All are licensed with Creative Commons or released into the public domain. Search by keyword, subject, or format.
The search bar is located in the upper right corner. When you click on an individual item, the copyright status should be provided in the lower right corner of your screen.
Pixabay provides millions of stock photos and images that can be used for free without attribution.
This site allows users to browse photos or images produced by the U.S. government. These works are automatically placed within the public domain. Browse photos by subject ("Science and Technology", "Health and Nutrition", "Environment, Energy, and Agriculture", etc.) or government agency ("White House Photo Gallery", "U.S. Fish and Wildlife Service", "National Institutes of Health", etc.)
The Metropolitan Museum of Art has labeled items believed to be in the public domain (free to use) with 
Keep an eye out for that while searching more than 400,000 online records. Limit your search to "Artworks with Image" to save some time:
Search this site to find more than 50 million digitized artworks, artifacts, books, videos, and sounds from across Europe.
Search for items by keyword, and limit your results by format, country of origin, or usage: 
Be sure to review the copyright statement that is included with each item.
The Library of Congress provides access to a large collection of historical photos, prints, drawings, maps, and other visual works. Most items are available for digital download and reuse. Be sure to review the Rights Advisory statement for each item you download.
The Getty Museum houses European paintings, drawings, sculpture, decorative arts, manuscripts, architectural sketches, and photography dated from ancient eras to the present. Through it's Open Content Program, the Getty makes available, without charge, all digital images to which the Getty holds the rights or that are in the public domain to be used for any purpose. Search for digital images in the Getty Search Gateway - all open content images are identified with a "Download" link.