The reality is that once an image is available on the Internet, [b]anybody[/b] can download it, keep it, print it, whatever. Such is the nature of the beast. Even if you try some clever things in coding a webpage (e.g. through Javascript), a smart enough person can defeat those measures and download your images. Now, this isn't really all that new -- anybody could clip a copyrighted photograph out of a magazine or book, etc.
What digital watermarking will do for you is help you prosecute infringements of your copyright. First by helping to establish that you are the copyright holder and second by actively finding instances of infringement -- where somebody posts your photo on their website. You can then see if they're using it with your permission or if they're in violation of federal copyright laws.
Unfortunately, you can't do anything about somebody who downloads your photographs to their own computer or uses them in a way that doesn't republish them somewhere (even though that's still illegal).