You are likely to just irritate your users using these techniques. If it's on the web, it available to whoever decides to take it. The watermark in PowerGallery is a good compromise because people can see it upfront so they they're less likely to get mad, than when things don't work as expected.
If you are selling images, keep the full resolution images off the web altogether. Nothing makes me leave a site faster than one that messes with my browser tools.