Jordan-
Without knowing more about the situation, it's difficult to render an opinion. However, if the photo is one that you produced, and they have placed their watermark on it, claiming it as their own, then I would at least pursue the matter with a C&D letter. I think it's quite possible (even probable) that they will immediately remove the photo from their site, because I think we'd agree they know full well that they stole the image. In that event, you'll have at least saved the money you would have paid a solicitor, which could be much better spent on some of your country's excellent ale!
If they refuse to remove it, or even respond, then you're faced with the decision of whether or not you want to throw money at it. As a matter of principle, I can certainly understand wanting to do so, but only you can judge the worthiness of the investment.
Good luck!