There are many sites that share stock images that have been licensed under Creative Commons licenses, which indicate how the images can be reused in plain terms. Search for CC-licensed images through these sites.
Other sites release images, usually for free, under their own license terms. For more information, explore the sites below. Pay close attention to the usage information available on each site.
You can filter many search engines to locate images. Within these search results, change the settings to retrieve images that may allow for certain uses. Remember, it is your responsibility to to learn the restrictions and permissions of each image.
The USC Libraries has an extensive collection of digital images that have been made available by the Digital Collections department. Other libraries and groups also provide access to images. This list is not exhaustive, but serve as a starting point for locating historical and special images. Restrictions may apply when using this content.