Everyone has used Google to find images, but it's definitely not the only source!
Flickr.com is a great resource for finding images that have been uploaded by museums, libraries, and individuals from all over the world. Many of these images have been assigned Creative Commons licenses, so you are allowed to use them in published works without first contacting the copyright holder for permission. Be sure, though, to use the Advanced Search option and click the Creatve Commons box. You need to recheck the Creative Commons box each time you do a new search.
Flickrcc.bluemountains.net will search Flickr for you and limit your results to just Creative Commons images.
Check this pathfinder created by Joyce Valenza (Springfield Township HS Teacher Librarian) for many great sites for images and more and this page wiki for many more sites for images.
LACMA's Image Library of Public Domain works
What's Creative Commons?
Check out this short, 2 1/2-minute video:
Imagechef.com is a great place to make all sorts of customized images and signs like the one above.
Wordle.net is a fun site for making word clouds.