Well I played with Digimarc today, and Fireworks, one of the most common image editing apps on the market. Not only, does it not support digital watermarking, BUT, you 'optimize' a watermarked image, and it no longer has the embeded information. YEEHA! for spending $79/year for something almost totally useless with the company that is supposed to be the kings of image protection!
I am thinking that one of the best ways to do this is to create a custom plugin or use a key system tied to a domain. So even if you snag source, you reveal the watermark. As for screen capture, I have that covered, besides the quality of a screen cap is pretty bad, but I have it covered in that eventuality.