Advanced Google Image Search: use these fields to hone your search; scroll down to USAGE RIGHTS field and select from the drop down menu. Note that the options include "free to use share or modify"
Creative Commons Search -- note the boxes to click or unclick for "use for commercial purposes" AND for "modify, adapt or build on"
Wikimedia Commons: enter search terms at top right, click on photo of interest in results list, scroll down to read LICENSING details. Copy, paste, edit an ATTRIBUTION statement for any content used.
Flickr Creative Commons Search: scroll down the page and select the type of licensed works you are interested in (Public Domain is the least restrictive), select an image, review the usage restrictions (at right, below image)
Unsplash has freely useable images made available by creators throughout the world. Attribution is recommended.
Many works (photos, videos, text) on the internet are protected by copyright and may NOT be re-used without permission unless "fair use" applies. Search for works with no restrictions or few restrictions (see box above).
Only use works which have clear attribution statements.
Educational Materials on Copyright Law from Villanova University's General Counsel
Fair Use Checklist from the University of Georgia
Crash Course in Copyright from the University of Texas
Fair Use Index: Search "Fair Use" cases compiled by the U.S. Copyright Office
Creative Commons -- Types of Creative Commons Licenses