When you are adding images, videos and other content that you did not create to your presentation, it is important to make sure that you are not violating anyone's copyright. One way to do so is to find public domain images for your presentations. Copyright.gov explains the public domain as follows: "A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner." Because such works can be used without first seeking permission, they are ideal for many projects, particularly those that will extend beyond educational uses.
Note: Even if a work that you use is in the public domain, it is advisable to provide attribution for the work or, at a minimum, keep a record of the attribution of the work, so that you or other interested parties can find it later if necessary.
If you can't find Public Domain media that fit your needs, you can also use Creative Commons-licensed content as long as you ensure that you correctly attribute this content to its creator and otherwise meet the terms of the license under which the image is offered. You can find more information about this on the Creative Commons FAQ.
Note: Even if content is covered by a Creative Commons license, you must always make sure that your use does not violate that license and that you properly attribute the content.
This video from CreativeCommons.org offers an overview of Creative Commons.
Check out this infographic by adityadipankar for a quick intro to the various types of Creative Commons licenses. If you are interested in learning more about these licenses, CreativeCommons.org offers more information.