Rewrite for image changing

[Gutspiller]

I have a image hosting site and it's getting hammered by hotlinkers. Fullsized image URLs look like this: http://www.dumpt.com/img/files/1esn8w0g20mmlddwdumo.jpg

Thumbnail URLs look like this: http://www.dumpt.com/img/files/1esn8w0g20mmlddwdumo_thumb.jpg

 

So the thumbnails just add a _thumb at the end of the filename.

 

I've heard that it's possible through htaccess to make it so people that call the fullsize images from their websites and forums are switched what picture they get so that instead of getting the image 1esn8w0g20mmlddwdumo.jpg they get the image 1esn8w0g20mmlddwdumo_thumb.jpg

 

Can somebody help me with this so that my site continues to show the full images, but that any other site that tries to call the fullsize of an image instead gets the thumbnail?

 

Thanks for any help that can be provided!