It's impossible to completely prevent your images from being stolen, but you can make the process harder.
The following method will make the real image unreachable unless checked from the source. You can use the original image as a background & put a transparent-blank file over it that matches the size of the real image.
<div id="image1" style="background-image: url(originalImage.jpg);"> <img src="blank.gif" height="250px" width="300px"> </div>
So, when the image is right-clicked, it will be the
blank.gif that can be reached.
View this post for some other ideas.
An alternative solution would be to use PHP's GD Library. Using this method, you can add a watermark on all your images.
Hope this helps!