How do I prevent visitors stealing my images?
Save As.., and so far none of them have worked, or worked in only one browser. These scripts can be defeated because they cannot cover all the entry points to the image. The browser belongs to the user, they can do whatever they please. Even, if by some miracle, a method was found to block the user from saving the image through the browser, there is still the unstoppable resource of the cache.
What about not allowing the cache to store the image? Think about it - who controls whether the image should be cached or not? Its the caching/proxy server that decides. You may request the image not be cached, whether the proxy server will abide by that is totally up to the user. Also preventing images from being cached takes us back to the stoneage of Internet technology - caching does a lot to speed up the Internet, because of local copies of oft-requested pages - they do a good service for your visitors
Technically, because of the foundation the internet is built on (TCP/IP), and the mindset of sharing information and resources, any programmed solution goes against the grain, and doesn't have much hope of working
But that's not to say there's nothing you can do to stop people taking your precious images.
A popular trick to prevent image theft is to "brand", or "watermark" them by putting your URL or logo into the image. Anyone who takes your images and posts them somewhere else will at the same time be advertising for your site.
For image galleries - don't use your big 1200dpi pictures on your website, only store images less than 72dpi on your webserver, and offer the user a way to obtain the more detailed image through online-selling, etc.
If images are your trade and livelihood, then you should be acquainted with copyright law already. You may want to contact your lawyer or solicitor.
Alchemedia makes a program called Clever Content (which required the user to install a plugin - so they have to be computer-savvy before they can see your images) which Microsoft's TerraServer service uses to protect the satellite photographs on its server that are licensed from private companies. If you try to take a screen capture of a Clever Content image, the capture will show a tiled Clever Content logo in place of the image. This can be very easily defeated by any software that can save the contents of the screen memory.
The rule of thumb is that if an image can be viewed on the users computer, there's nothing you can do technically to stop them copying your images, legally you may have a few options.
The second rule of thumb is that if your images are that important to you, don't put them on the net.