There have been many questions recently regarding how to find images to use in instruction and post in course websites (ie copyright-free images). Below is a collection of sites where you may find such images.
NOTE: Use of images & figures from the course textbook: Check the licensing agreement that your department signed with the publisher of your course textbook, you may be permitted to use textbook content in your slides.
Understanding Science Image Library - The University of California's Museum of Paleontology created this wonderful resource for teachers, students, and the general public to learn about what science is. Images in the Image Library can be reused with attribution to this resource.
The New York Public Library Digital Gallery has an impressive collection of digitized historical photos and other images. Terms & conditions: Low resolution files may only be used for personal, educational, or research purposes (not for commercial purposes). See these collections in particular:
All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.