I had this situation last year when I noticed a local not-for-profit group seemed to have hi-jacked five of my pictures from PoE, for use on their own website. Before I went out with all guns blazing, I was careful to make sure that the images really were mine by noting the matching features on the images eg shapes of clouds, shadows, times on clocks etc. What really riled me was that they had put their own copyright on my pics !!! 
My first action was to contact the Webmaster of the offending site pointing out the images were copyrighted, and drawing their attention to PoE's Terms & Conditions. It was initially difficult to get a response from the Webmaster, but eventually I got a telephone number and explained that the images were copyright and any use (even for not-for-profit) required acknowledgement both of myself as the photographer and PoE as the site owner. The Webmaster told me that 'someone had found the images for him off the web', as if this let him off the hook. I was not impressed.
To be fair, once they realised that PoE's T&Cs required acknowledgement of the source, they displayed my name and PoE as the source on each of my images on their website.
Maybe I was lucky in that the organisation involved was local, and I knew where they were located ! In the case of unauthorised use of images by an overseas website, it is obviously more difficult, but I would try contacting the Webmaster of the offending site in the first instance, and see if this gives any joy.
PS While writing this post, CathyML's next post came in, and I see the images were taken off Flickr...not sure what Flickr's T&Cs are......I assumed that the pics had been lifted off PoE
Edited by: Edward Lever at:27th April 2012 23:28