Here are some great repositories of digital media in a variety of formats.
Many images are not cleared for use unless they contain a statement that the creator has given permission. To make it easy for this to occur, the Creative Commons licensing was created. See more information here: https://creativecommons.org/about/
In addition, there are many image search options to locate these types of images - those that can be reused with permission already given. Here are some good places to look:
Flickr for images: https://www.flickr.com/creativecommons/
Here is a listing of other good sites: http://budgetstockphoto.com/creative_commons_images.html
And here is a great listing of Public Domain sources: http://guides.library.harvard.edu/c.php?g=310751&p=2072816 (plus some good creative commons sites too.)
Search the Creative Commons site. Please make note of their warning at the bottom of that page.