Do you know any site that is hotlinking picture from your site?
Actually, you can do ‘cool’ things to them…
You can use this opportunities to make fun to them. Just the pointed picture at your server to another picture.
If you wish to promote your site, change it to a picture that has the name of your site.
Want some “better” idea? Turn the picture into porno picture… Ha, ha. Hope you don’t this.
*Sometimes I like to hotlinking too, hope that they won’t do this to me 
If you really do not want other site to make hotlinking, you can block the site at your cpanel. Hotlinking is bad because it use your bandwidth but not provide incoming traffic, but I think it is not hurt so much.