I would never see it as flattery, if someone stole one of my photos. That's exactly what's happened here. If it were my photo, I'd demand it be removed and I'd do it very publicly, such as on their Sponsor Forum. If they refused to remove the photo, I'd take further actions. Doug is right, in that you can't collect any monetary damages, if your photo hasn't been registered. However, if you demand removal of one of your photos, and removal doesn't take place, there are other damages that can be rewarded.
There's no such thing as Public Domain, in photography, UNLESS the photographer specifically allows their pictures to be put in the Public Domain.