Wouldn’t it be far easier to add a “generic” watermark with the forum name ?
As you say, @BishopV, usually a watermark prevents people from sharing the image (at least publicly), and in any case, prevents them from claiming ownership/being the author.
I guess that anybody a little clever and wanting to get images to spread them, would create another user just for that purpose. So, you are willing to go a pretty complicated route for probably no good reason. A generic watermark seems a good solution to me.