If the image is hosted on their forum/site and shows up etc you could possibly go to google and file a DMCA request - Digital Millenium Copyright Act and demand it be taken down as the images rights are owned by you.

This would likely be taken fairly seriously by the site but ultimately there isnt much you can do.

I suggest, as much as ti sucks, moving on and making some money rather than getting annoyed by what is obviously a really frustrating user stealing your pic.

best of luck