>> I want to stop the user from saving the image to local disk?
you can't prevent _any_ picture that is displayed on a computer monitor from being saved. Just press Ctrl-Print-Screen, go to MS Paint, and press "paste".
Let's put it another way;
a) clearly in order for an image to be displayed in a browser it has to be downloaded from the server.
b) if the person inspects the html, they can fetch "just the image file". For example;
http://www.google.com/images/srpr/logo3w.pngc) if they can fetch it with a browser they can fetch it with any "web client" software, like wget or curl or indeed many of the NetTalk WebClient demos.
Cheers
Bruce