If you review your web site statistics you may see that your web site's images are being found through Google's Image search (http://images.google.com/). This in itself is not bad. However, what sometimes happens is that people come across an image on your site and then link to the image in a blog post, online forum post, etc. which causes your bandwidth to be used.

To prevent Google from indexing the images on your site, add the following to your robots.txt file:

User-agent: Googlebot-Image
Disallow: /

This will tell Google's Images spider to not index any images on your site. For more information, see Google's information page. If you don't already have a robots.txt file, just create a file named robots.txt using a text editor and add the contect above to it. Save it and upload it to the root of your web site.