Got it.
I do think it brings up a usability question. If I as a user wanting to share a topic from my phone, simply grab the url, then I imagine other users will do the same.
Obviously, much bigger things to address, but I would say that the image from the first message of the topic would be better than that of the posters avatar. Though, the user experience would still not match my intention, since users would bypass the images at the top of the topic