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.
Some webmasters try to disable right click on their webpages 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. Image Maps are another option but the sliced images can be saved as a whole using any screen capture program.
So what's a good alternative if you are very eager to protect you images on the internet ? One option is is convert your images to SWF Flash objects before uploading them to the web but if that sounds too impractical, try swfIR (SWF Image Replacement tool).
swfIR is a very simple technique to prevent image download from your website - instead of adding the standard tag for embedding pictures, you define the image location in Javascript and the image will then wrap inside a Flash movie on your webpage.
What's so nice about swfIR is that it lets apply nice visual effects to your images including borders, rounded corners, small rotations and shadows without actually modifying the original image.
Tech savvy users can easily download the image by looking at the HTML source code of your web page but for the not-so-geeky crowd and image leech software, all they'll see is a blank Flash movie.