I recently had someone ask how they can protect their proprietary images from theft on their web site.

Technically it is impossible to protect web pictures from leeching since most browsers will first download a copy of the entire image from the internet to the local cache before rendering.  Image slicing as you mentioned below helps, but can be tedious.

Some webmasters try to disable right click on their web pages so that visitors cannot access the "Save Image As.." command in the menu but that trick can be easily defeated by disabling JavaScript or using Firefox.  Screen Capture programs also get image thieves around the disabled right click as well as the image slicing.  It really becomes a matter of making it as difficult as possible for them to get the image.

Another method to discourage people from downloading your images is to put a light watermark on images posted on the web.  The issue with watermarks is to ensure their opacity is high enough to be visible, but low enough that people can still see the quality of the original image.  You can create your own watermark using software like Paint Shop Pro or Photoshop.  There are also a number of software packages on the web as well (Google search) for watermarking images.

I would love to hear what other web masters have done to protect images.  Feel free to register and post on the forums.