Not everything you find on the internet is free to use. When you searching for images and other media, keep these things in mind:
Google Images allows you to filter by the type of usage rights associated with an image:
Creative Commons: Creative Commons licenses give creators the power to set specific guidelines for how their property is shared and reused. Creative Commons licenses will either state that it is licensed for reuse under a CC license or will have a CC marker like the ones below:
Find out what each of these symbols means on the Creative Commons website.
Public Domain: Public domain materials are owned by the public, and not protected by any intellectual property laws. Anyone is free to use public domain materials with out permission or restriction. Often, materials enter the public domain because the copyright has expired, but there are a number of avenues that can lead to materials entering the public domain. You can read more about the public domain on Stanford Library's copyright and fair use guide.
Below is a list of a few sites with Creative Commons-licensed and/or public domain media.