Protect images

[Britmovie]

Hi i have noticed that some sites that have pictures linked to other sites show the pic then it changes to a pic saying stop pinching our bandwith. This was on a vbulletin site. How is it done so i can do it on my site?

Or whats the best way to protect images?

[Stickers]

http://www.clockwatchers.com/htaccess_images.html ;)