bender is right...if it can be viewed by a user, it can be downloaded/saved. that includes images, videos (youtube and any other site that uses flash players, etc...)
javascript is never an option for security, as you can simple disable javascript in your browser.
you can restrict people from linking to your image from an external site in order to save your bandwidth, and you can add a watermark to the image to prevent people from passing your images off as their own, but that's about it.
Good luck. If you need any help watermarking, just ask