Having mulled over various options, I came to the conclusion that asking the poster to give me credit was pointless.
Nobody had viewed that post in 8 days or so, and I knew nobody was likely to now.
I chatted to a friend and he talked about blocking access with Apache.
We looked at the Apache site and eventually found this (you have to scroll to the bottom of the page).
It’s called Prevent Image Theft and it basically allows you to prevent people accessing a file if the referrer wasn’t from your site.
Appplying this config change to my images completely blocked my photo from appearing on his post while not affecting my site at all.
It’s not a perfect solution but it works in this situation.