We have a similar discussion going on here: [webmasterworld.com...] - except that thread discusses hotlinking effects on safe-search filtering in image search. The fixes being discussed there would be the same as you need, esentially break the hotlinking.
The last time this happened to me I put an ad for my site at the old image with something like, "for better prices visit www.mysite.com" and then I renamed my old image and linked to that the old image with the new name on my site. They stopped hotlinking to my images pretty quick after that.
Another time some foul mouthed gamer guy from a quasi porn forum site with a lot of page views hotlinked to one of my images for his avatar in his forum posts. I renamed the image on my site put a religious one in its place. I don't remember exactly what I put but he had something like a cross or a prayer book as his avatar for a day or so. He changed his avatar pretty quick and then I changed my image back to its original name.
If the people doing the hotlinking have their sites on a place like Ebay, you can rename your image on your site, put an adult picture under the old name and then report them to Ebay for having inappropriate images on their ads.
I experienced this too from the other side. I linked to a pic of Madonna from a major national magazine (not exactly a small niche site nor a particularly original image) and got a bunch of image search hits for it. It was sort of a halloween related post so they have dissipated.
I think my page must have had some attractive keywords that the original didn't.