Many search engines (e.g. Google) have an image search available. Most images on the web are copyrighted but may be available to use for educational purposes. To locate images that are "free," "copyright free," or in the "public domain" use images from the sites linked to the right, or try using the following search terms in combination with one another:
public domain or copyright free or royalty free + images or photos or graphics
Or, search for your topic using Google, click the Images link, click the Tools button, then click Usage Rights. Choose any of the license options (other than the default "Not filtered by license") to display images that may be used for educational purposes.
Find more information about copyright on the Using Media tab.
Online image collections that permit educational fair use.
Library subscription databases for images.