I am guessing you would like to use images for your website that are legitimate. If you don’t care about legitimacy, you can use any image you find!

For the rest of us, there are two great resources. (Note that the links are at the end of this post.)

Flickr is part of the Yahoo family and has more than 25,000,000 photos that you can legitimately use without any charge. The only condition is that you credit the photographer.

For historical photos and illustrations, Wikimedia Commons is an awesome source. And most all of their content does not even require attribution. (Be sure to check the license section on the page of each image.)