I think what Cthulu_ says is to serve clean and nice pictures to people coming through your server, and instead serve heavy watermarked versions for hotlinked links.
I'm not sure how this can be arranged with a CDN, but it's probably relatively easy/there's a painless way
yes, I understand. But even if technically possible, it won't really solve our problem. It might solve hotlinking, but not copy&pasting our images / cropping them and uploading them elsewhere, which happens probably just as frequently if not more frequently than just using our own CDN link.
Furthermore, there's a small hidden benefit of people who do hotlink to our CDN - it makes it potentially easier to trace.
Copy&pasters would require a more sophisticated search which is either labour intensive to do manually for us, or costs money to do via API.
My main concern is not about technical aspects, but rather legal, ethical and practical. Primarily, how to deal with sites that rely on user-generated (or user-stolen) content where offenders might keep on doing this...
Register a LLC in the US, which is not expensive at all, this should get you on the road to sending DMCA takedowns to at least the US sites hosting your content. Here on HN, search for the user named grellas, who is a lawyer and may be able to guide you in that regard.
Disclaimer: IANAL
Nice site! Like the illustrations, had fun brushing up on things from my medical school days.
I'm not sure how this can be arranged with a CDN, but it's probably relatively easy/there's a painless way