Blocking Right Click or the Context Menu
One of the simplest ways to help prevent your images from being downloaded without your permission is to put up a no right-click script. When people right click on your page, they will either get no options to download the image, or they will get a pop-up error message (depending upon how you code the script).
This is very easy to do, but also easy to get around.
Shrink Wrapping Images
For someone who is determined, this method can be circumvented as well.
Watermarking Is Fairly Effective
Watermarking is where you place an overlay directly on the image. This usually impacts the quality of the image such that potential thieves don't want to steal it. This is a very effective way to protect your online images if you don't mind the text across the top of them.
Using Flash Can Protect Your Images
It is also possible to set up a slideshow in Flash to display your images. This makes it impossible for thieves to download the images directly.
But, Fully Protecting Your Images is Impossible
If you post your images online, it is possible for someone to steal them and use them somewhere else. No matter what you do to protect them.
No right-click scripts can be defeated by using a view source bookmarklet and browsing to the image directly. Shrink wrapping the images can be defeated the same way.
Watermarks can be removed (with difficulty).
Even if you embed your images in a Flash object or something else to protect them, it's possible to take a screen shot of the desktop as that object is displaying your image.
If your image is so valuable that you want to be sure no one ever steals it, then don't post it online. That is the securest and safest method of protecting your digital images.