@zeus: thank for the htaccess-advice. I have installed that a while ago - works fine. But how to find ou new "bad guys"?
Edit: Ok, sure, the answer is: look directly in your log-files for the "bad-referer". But this is hard work ;-)
@leosghost: I agree with that, but most people didn't see the problem. 90% (?) of all people are not webmaster or seo. They use images in forums or facebook or whatever just for fun. Often they find it via google-image-search.
So as creative artist I agree that hotlinking is illegal.
But: it is actually a strong ranking-factor. Google loves hotlinks. So I have to decide which hotlinks I like and which not.
The border between legal and illegal ist very blurred. In germany we have e.g. some "personal search engines" that use the google-thumbnails on their pages to set a hotlink.
I am not a lawyer but this seems to be very tricky...