Welcome to the way of the web. There is no way to actually stop someone from stealing your images. If they can view them, they can steal them. I get asked the same question on about every third Zen Cart system I install and even more so from standard website clients. I've tried every way known to protect images of intellectual copyright such as photos of specially designed products and NONE work.
In the past I have tried, disabling right click, loading a transparent image over the top so that the blank image is the image saved, loading images into a Flash player and various other procedures. None of them will stop a dertermined image thief from copying your images and in fact some of the options will actually deter your customers.
Your best bet is to watermark all of your images so that if they are stolen they can't be re-used. This won't however stop them from copying your designs or your ideas.