You can't: if I can see it in my browser, I can download it to my hard drive; period.
You can try to use flash, disable right-click, obfuscate the links, and many other alternatives; some of them will be harder to by-pass that others, but the crude fact is that any image than can see in a screen can be saved to a hard drive and uploaded somewhere else.
My two-cents piece of advice is to forget trying to avoid it: all you are going to do is lose time and money, and probably even piss some legitimate visitors. You could add a watermark to the images, if you feel like it (I don't, but that is just my personal choice).