IMPORTANT: Use of images is subject to U.S. copyright law. If an image is copyrighted, you must ask permission--and often pay--to use it with an accompanying credit line. That's the law! If an image is in the public domain or is published with a Creative Commons license making it freely available to use, often it will say so and provide credit information.
If you are not sure if an image is public domain or Creative Commons, contact the person or entity responsible for the image and ask permission for one-time educational use with an accompanying credit line. For more on copyright, see U.S. Copyright Office FAQs.