Pixabay provides millions of stock photos and images that can be used for free without attribution.
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.
This website provides links to many government image collections. Some of the photos are in the public domain or U.S. government works and may be used without permission or fee. However, some images may be protected by license or copyright. You should read the disclaimers on each site before using these images.
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 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:
Unsplash allows you to browse 3 million free high-resolution images that you may download and reuse for commercial or non-commercial purposes, with or without attribution.
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.
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.