Today, I caught sight of an article on anti-plagiarism tools at The Blog Herald and among the tricks was to use mod-rewrite to stop people "borrowing" both your images and your bandwidth. The gist is that you set up one or more conditions that exclude websites from the application of a rule forbidding access to images; the logic is that if the website referencing an image is not one of the websites listed in the conditions, then it doesn’t get to display any of your images.
RewriteCond %{HTTP_REFERER} !^http://(www\.)?awebsite.com(/)?.*$ [NC]
RewriteRule .*\.(gif|jpe?g|png|bmp)$ [F,NC]