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.
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.