If I were REALLY worried - for whatever reason - about having a public image 'stolen' (there's also no point in protecting something unless you also print a copyright statement on the image or next to it) - is very simply this. Edit the jpeg header to contain copyright information. Or create a footer - and this way if you find your image being used unfairly - well, not only do you have your original film, but you can just open the image with Word or whatever text editor and then - voilą! There's your name. Of course, this will only work with direct copying. The info would be lost via screenshot. But if you keep the image freely copiable - nobody's going to bother with the screenshot and your information will stay with the image.