There are plenty of good places to find public domain and Creative Commons images to use in your Google Slides presentations. The Unsplash photos add-on even makes it possible to find public domain images without ever leaving the slides editor. But even with the wealth of images available in the public domain, using your own images can be your best option. If you use Google Photos to save all of the pictures that you take with your phone, you can easily add those images to your Google Slides presentation. Watch my video that is embedded below to see how easy it is to add your Google Photos images to your Google Slides presentations.
On a related note, here are three good places to find free images to use in any multimedia project.