It's a tricky situation...

1. If they have downloaded your photo and have re-published it without your permission then you have a viable complaint.
2. However, if they have just linked to the image on your server from within their advert (this doesn't have to be a click-to-view link, you can display the image by linking to it) then technically they haven't republished it at all, and you can't legally do anything about it.

You can configure your webserver to not serve up images unless they are linked to from pages on your webserver to stop the second problem.