With that said, you can disable the menus in Internet Explorer and other Java-based browsers. I did a quick search on Google for the phrase "disable 'save picture as'" and found several sites that gave Java code that could be inserted into your page to remove the right-click menus. Click here for results to that search.
As I mentioned, this is not a perfect solution. The user's browser still stores the image in the cache, which generally means the file is written to the user's hard drive. The user could examine the browser's cache and pull the file directly from the hard drive. This solution also depends on the HTML page in which the image is referenced. Therefore, if the user places the URL to the image in the browser window, that user will bypass your Java script and be allowed to right-click on the image. Another method to save images is practically impossible to defeat -- the user could press alt-PrintScreen to capture the entire browser window to the clipboard, then paste the capture into a program such as Microsoft Paint.
A non-technological way to protect your images is to explicitly state their copyright. You can either add that text to the page from which the image is referenced, or use watermarking to add text directly into the image (for example, "Copyright 2002 John Doe").
This was first published in November 2002