piatkow - 10:02 am on Oct 4, 2012 (gmt 0)
Mostly from other sites that are scraping mine.
If they find your picture hotlinked they will return all the pictures on that page when you do an image search search on your domain name.
My impression is that they give more weight to the full url in the hotlink than to the relative addresses on my own pages. I haven't seen any of these sites come up when searching on keywords.
I doubt if it was Google's intention but, given that Joe Public is most unlikely to do image searches on our domain names, we have a useful tool for a one-shot identification of hotlinkers.