Preventing the "hijacking" of pictures

Hi there, I recently discovered the reason why the traffic is rising so high on my server. Some people are "stealing" bandwidth. They include the link of an image into a forum posting. Now everytime somebody reads this threat on the other site this image is served by my server!! No one cares about one image, but they become dramaticly a lot! Is there a reason how to find out that the image is not include into my html code, but into the other site code? A great idea would be showing a kind of a watermark on this images if they are included inside other websites. $_SERVER[HTTP_REFERRER] does not help. But there might be another method I do not know of. Thank you for any help and hints on this topic! Merlin