It depends...
Check for copyright. Copyright protects the image creator, however, you still may be able to use an image. The Fair Use doctrine allows limited use of copyrighted material for educational (vs. commercial) purposes. Do give credit to the image maker!
If an image is in the public domain, it means that it used to be under copyright but that copyright has expired OR that it never met copyright condition. You are free to use the image. You should provide a source just to be safe.
Creative Commons images are not always copyright-free, but they have licenses that clearly state how they can be re-used. Note that Creative Commons licenses vary. All licenses require that you attribute the image. See Flickr images with Creative Commons licenses. (You have access to these in Canvas.)
Whether an image has copyright or not, and even if it is in the public domain, you should credit the source of the image--the image creator or copyright or license owner. If an image has a Creative Commons license, usually, you are required to include a link to the license if you re-use the image online.