You can use digital watermarks. These are one pixel areas of the picture which can be tracked by online companies, normally for a fee. If your pictures reappear elsewhere on the web, and u have the rights, then you can force them to take them off.
You could just copy another bit of grass instead of the altered one. Or cut it out, because you wouldnt want it too big. Oh and i was just flicking through, but in Dave's script, shouldnt there be parantheses after
For a watermark, just have your site/copyright written nice and big across any of your images, but just lower the opacity to where the text is faded enough to where it doesn't interfere with the pic too much. I was also thinking about the idea of just adding a layer across the whole pic of a certain color and lower the opacity of that layer. This way the colored layer covers the whole pic. Not sure if that makes it harder, or easier to undo, but at least doing it either way it's going to be a pain to undo.