Wikimedia has over 15 million photos, videos and sounds that are all public domain or freely licensed for use. Like the other Wikimedia projects, the content is user provided and curated.
You can search for images from the Wikimedia Commons page, or by checking the useage information for a file on a Wikipedia entry page.
Once you find an image you would like to use, choose the correct version of the file here and you're all set! Make sure to cite it appropriately, even though you can use it for free, you still need to give credit.