[Logician]

While you take the precautions mentioned above, have some fun for a while:

Replace that attachment (probably a picture?) with an other picture of your site banner or logo. Or if you want, you can put a picture there mentioning "This picture is being stolen from Site X" etc. Will give him a good lesson.

BTW. You can post your original attachment after that one.

If your hosting company provides you enhanced log files, you can track the referral of the visitor to find out which site uses your picture.

And don't forget prevent hot linking for an effective solution.