While linking to other peoples' images may be rude, it is legal. A couple of years ago ticketmaster took tickets.com to court over "deep linking" which is essentially the embedded of whole pages, not just images, from one website within the frames of pages on another website. Ticketmaster lost the case and thus there is now legal precedent, at least in the USA, to support linking stuff into your pages from other people's webservers.

And really, the technical solution is easy and doesn't require a whole lot of scripting. There are a lot of tools that make it simple to filter requests for images (or any other file type) by referrer. Even simpler is to keep an eye on your log files and if you see your images being abused, just rename them and your own links and then put some nasty-ass goatse.cx pictures in their place with the original filenames.

Trust me, nobody is going to sell an empeg with the goatse.cx guy as the image of their product. It would probably be enough to get the seller banned all on his own.