There are ways to search for Public Domain images and Creative Commons images. Public Domain means that you can use the image without having to give credit or attribution to anyone. Creative Commons means you need to give attribution to use the image.
Photos for Class is a great website that allows users to download high-quality images for free and not have to worry about copyright. This website uses Creative Commons Images. This allows users to use them without fear of violating copyright law. The user information is added at the bottom of the image automatically for you. All images are G Rated so they are safe to use in school.
Pixabay is a website that allows you to search for Public Domain images. These images are free to use however you wish. This website is not for schools necessarily. The images are not screened to be safe for schools like Photos for Class. However, you don't have the attribution on the bottom of the images. You just need to teach your students how to handle inappropriate images.
Google Images can be searched for Public Domain images, as well. You need to go to Google Images and search for the topic you choose. Then click on "Search Tools" and then "Usage Rights." If you click on "Labeled for Noncomercial Reuse with Modification" to get images that you can change and use for any reason.