Don't send me hate email for not looking out for the 'little guy', but I believe that unless you're a wealthy, successful artist (or have good journalist contacts who can publicize the matter for you), it'll probably cost you more than you'd get from suing for copyright infringement, but that's not to say you shouldn't try. Consult a copyright lawyer to see what your options are (just remember to ask what their hourly rates are beforehand), approach the company/individual(s) involved, and consider your choices in the situation.
If someone has copied material/photos from your website, start by writing to them pointing out exactly what items have been used without you permission and ask them to be removed. If they refuse, write to their internet service provider, providing a list of the URLs where the material is (on the offending site and on yours), asking for it to be removed.
If you want people to be able to use some of your material, or in specific ways only (such as non-commercial or no derivatives), investigate the Creative Commons range of copyright licenses which sets it out in clear, unambiguous language.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.
