Hi JTFOTO.
In Europe, or in my part of Europe at least, it is not necessary to register copyright, you automatically have it as you created the image. Noone is allowed to use an image that is not theirs, just because it doesn't have a copyright stamp on the back.
Assuming it is a digital image and that you have the raw file it would be fairly simple to prove that the image is yours.
I would contact the company and tell them that the image is yours and that you get paid to supply images to companies just like them and that a use of your image has a price. You can then ask them (politely, as they could become clients) if they wish to continue to use your image for a price or take the image down (ask them in writing, email is fine, ask to get a receipt of them recieving the email). It may be that they are simply not aware of how things work...I have come across some examples of really clueless, but essentially nice, clients.
If they ignore your email, write a letter asking them to take down the image or you will take them to court, contact an attorney first and ask for advice.
A good way to help customers 'find you' is to add you contact and copyright information to the meta data of all digital files. Thus allowing people to ask permission to use or buy the image from you. But if they are really clueless, they won't know how to read the metadata and you are back to square one
Good luck.
Anders