I have a friend who did that. He operated a website and noticed from the activity reports that his ISP provides that his site was being hit an unusual amount from a University's computer. He was able to do some backtracking and found that a student had linked a file from his site to his own web site. Every time someone accessed the students website the site would then access my friends and retreive the image for the other guys homepage. Over a holiday when the university was in recess, my friend changed the image attached to that file to something really embarassing and edited his own site to correct for it. Of course as soon as classes reconvened after the holiday the link had been changed. It appeared that while the culprit was home for the holidays he didn't have access to a computer to fix the problem.

Of course this only works when the violator links to your image file on your ftp site. It would not work if they copy and save your image through right clicking it and saving it in their own ftp.