Freepik https://www.freepik.com/ is a repository of images, graphics, and photos which may be useful for learners and teachers in making their resource materials more visually appealing. Each image has different licenses in terms of copyright, but most of them are available for attribution.

A more refined search for themed and quality images is available in https://www.freepik.com/stories

For easier searh, you may search keyterms+freepik (ex: learning+freepik) in Google Images search.