Here is an excellent resource for free clipart. Open Clip Art Library (http://www.openclipart.org/) allows anyone to use the clipart for anything. If you do use images form this site, they ask you to provide the URL (http://www.openclipart.org/).
There are over 6900 images from over 500 artisits.Once images are submitted to the project, they become Public Domain. You can do a keyword search as well as browse the catalog. You can also download the entire catalog for offline use.