There are many places online to find free images that you can use. To be safe, remember to always double-check the licensing conditions of individual items. Here are some sites that we recommend:
Openverse, formerly Creative Commons Search
A meta-search tool that searches across more than 300 million images (and soon also audio and video). Can be used to find CC-licensed and public domain images on Google Images, Wikimedia, Europeana, etc.
Flickr Creative Commons Search
20 million freely usable image, sound, and video files. All files are Public Domain or CC-licensed. Check the “Licensing” section for an image to find any terms and conditions for reuse / instructions on attribution.
UBC Library curates a great list of places to find Openly licensed and Public Domain materials. Pay attention to the different columns of use: whether in the LMS (Moodle) or on a public site.