If you don't want people to save your images or to have them for personal use, then don't post them on the web.
Simple answer to this question is there is ALWAYS a way for someone to take what they see in their browser window.
You can make it more difficult for them however. For example, you could add borders to the images that make them blend in with web site design.
People would be much less likely to snag this image:
Whereas this one would be more useful to people:
Or maybe better yet, just slice up your images. Like what I've done with this one. Makes it a real hassle to save the actual image.:
Of course you can't stop them from doing a print_screen, but that produces a much inferior, poor copy of the image, usually.