Be sure to limit to images that are openly licensed for reuse; see the Creative Commons page for more information on licensing terms that are frequently used.
Wikimedia Commons is a media file repository that is completely dedicated to providing educational media that are released either in the public domain or under Creative Commons license.
Flickr hosts millions of Creative Commons-licensed images. Notably, Flickr provides a separate Creative Commons licensed videos collection
Google Images provides a filter to limit to creative commons images.
Bing Images lets you limit results to creative commons licenses.
Getty Open Content contains all images of public domain artworks in the Getty's collections. These high-resolution images of the Museum's collection free to use, modify, and publish for any purpose.
Source: "Images" by Open Washington: Open Educational Resources Network is licensed under CC BY 4.0