Therefore a water mark would spoil the fun for the thieves

I know there are ways to remove those too, but that would be a bit more involved. If you create the watermark in say Photoshop, I wouldn't upload the .psd file.

Why not sell a PDF with all the content in it and just put up a image of a pdf with a picture of a Bonzia on it. The trick would be in explaining what you have for sale not showing your system. By showing a pdf booklet you are selling a full explanation. you could show images of stages of development of the plant not its controls.

I sell software, true I am paid before a copy is sent but nothing really prevents copying. All the above suggestions are valid. Another thing is to offer support to those who purchase. One thing that prevents intellectual property theft is in not asking to much for it in the first place. A thousand copies at $2 is better than 20 at $15.

Look at CC they openly make software they sell not only available 24/7 they offer upgrades for life. Why would anyone try to get copies underhanded without such a support system in place.

Another thought why not give the first part of the form free with the rest covered. It will encourage someone to start and they will come back to you to finish. One last thing
"Copying is the best form of flattery" You must of seen my clones wondering around all over the place

I hear a lot about screen capture "software", which I've never used. Instead I use the little "PrintScreen" on my keyboard ("PrtSc" on some keyboards).