There are three kinds of free images, royalty free, which means that you pay a small up-front fee and are allowed to reproduce the image either in a printed work or on a website without paying any future fees to the copyright holder, library free images, which means that you can browse through and use for educational or personal purposes for no charge, but which would have a royalty payment due to the copyright holder if you wanted to use the image in publication, and then completely free images which would have no up front and no royalty payment.
The last category is awesome, when you can find it, and you can find a bit of clip art of this type by using Yahoo! or Google's image searches.
For the second type, you can get CDs with tens of thousands of images on them for <$10 which I suppose isn't free, but darn close. I saw a set of 150,000 images for $6.99 a couple of days ago, though I can't think of where that was... possibly the gas station...
And for the first type, which is probably a smart idea if you intend to make money off your website, you can try a site like sxc.hu or bigstockphoto.com or even clipart.com -- it would suck to set up a successful business and then discover that you picked an image that someone else owned a copyright to by getting sued over it.