Gary, Nemanja is right on target...see my earlier thread on
How to Respond to Difficult Clients.
You can watermark your images but that will not keep someone from taking them to another shop and using them. Drawings are considered the property of the creator and have some copyright protections, but full copyright protections are available only if the drawing is registered with the Patents and Copyrights Office.
Ask yourself this: How many times has a client come to you with a magazine and said "I want this kitchen" and you, in turn, used the picture to design a kitchen for the customer? This is exactly what you are trying to prevent and it just can't be done. You would have to spend an enormous amount of time (and money) attempting to enforce a copyright. Unless an image is worth several million dollars in
ongoing income (such as an artist selling prints), its just not worth it.
The cost of registering and defending copyrights for images (in research and legal fees) is not worth it in my opinion. It is better to change your business model and charge for design time. If you choose not to charge for design time, be prepared for every potential customer to take drawings to another shop.