I'd advise you to file a DMCA complaint with their host. They are legally obliged to remove the content. You can also file a DMCA complaint with Google and get them kicked out of the search results and a DMCA complaint with the Google Adsense team and get them kicked out of the Google Adsense program if they are in it.
However before doing this, try writing to them and asking for an explanation. There have been cases where people have purchased images from stock websites where the seller has been illegally selling copyrighted images i.e. they may not have stolen them as far as they are aware.
If they have taken your images illegally, you've got quite a lot of leverage over them as you can see. Maybe possible to send a large invoice for unauthorised use of your images.
Also be 100% sure they are your images - if you end up screwing somebody up and you are not in the right they can sue you. Also make sure you can prove they are your images - do you have the RAW files, proof of upload date (check archive.org for history of your site and their site) etc.