Creative Commons
Creative Commons is a group of licenses which enable use of many online images, videos, music and other digital materials. It is a great resource for academic institutions as most works covered by the license allow for use in an educational setting without the need to contact the copyright holder. There is also a Creative Commons license that allows use for any purpose. Note that when using a work covered by Creative Commons you must always credit the work.
To search for images covered by Creative Commons licenses use the CC Search
Licenses
Creative Commons gives access to materials under a number of licenses. These range from licenses which allow usage for any purpose, to those which are limited to non-commercial use.
You can get more information about Creative Commons and their licenses on the official license page.
Image Resources
Pixabay: https://pixabay.com/
Pexels: https://www.pexels.com/
Lots of CC art - including from major museum collections - is here: https://search.
Here's a UK CC source - but we must use the CC filter when searching: https://artuk.org/
Tons of stuff here, but check each linked page for directions to get only the CC content for each platform: https://guides.lib.