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.
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.
This infographic by adityadipankar provides a quick introduction to the various types of Creative Commons licenses. If you are interested in learning more about these licenses, CreativeCommons.org offers more information.
