IMPORTANT: The 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 is the law. If an image is in the public domain and thus freely available to use, often it will say so and provide credit information.
If you are not sure if an image is in the public domain, 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 the U.S. Copyright Office FAQs.