Defeating scripts like that is no mystery... it's just that it's not exactly common knowledge. Some work better than others and that particular one can be defeated just by keeping the right mouse button depressed while canceling out the pop-up with the left.

All you can do is to try and make it less easy (for want of a better phrase) for your visitors to take them or make them less attractive by adding you site URL on an important part of the image. You're never going to stop everyone from taking your images but you will stop a few that way.

My own favourite methods are to put a transparent image over the top of the main image so that they save nothing, or splitting up the image into small sections with a program like Splitz... http://www.b-zone.de/software/splitz.htm That way they'll just save a little bit of the image unless they do a screen grab... and most people wouldn't even know that they hadn't got the whole thing until they went back to the image later.

Just my 2 pence worth :)