You can’t really. Virtually every measure you can take is reasonably easy to get around.
The best thing is to watermark your images, and then upload only low resolution, smaller images. I never upload anything I’m concerned about at more than 800px wide, and if I’m really concerned about it, I’ll only upload something around 400px. 400px at 72 DPI is not of much use, and if they try to enlarge it, it will loose so much quality as to be useless.
If you really, really don’t want your images stolen, then don’t put them on the internet.
You can use the (very limited) Duotone or Monotone themes; they disable right-click,save on all but the left-hand side of the latest image. It’s easy to get around, but really lazy thieves won’t bother, they’ll just steal from somewhere else.