I have a site where much of the focus are the images themselves. When people visit the website directly (with an AdSense ad shown), the image shows in normal high-quality. 

When robots visit the image, they will see a watermarked version. The reason for this is because the photographer of the site dedicates his time in taking high quality pictures and it would be nice if he could profit from it.

What I tried for a month straight is to have the watermarked versions of the image indexed with Google so that when a user clicks on it, they will know more about the website since its stamped on the watermarked image. The problem is not a single watermarked image is indexed.

What I did in the past was allow everyone to see the image the same way (unmodified, no watermark, and high quality), then all the images were indexed. I learned doing that is a mistake because then people could simply download the images right off Google without seeing my website or even an ad.

In both cases, I have submitted sitemaps pointing to valid URLs and I did not use any noindex or noimageindex robot tags and I did not use *robots.txt* to deny access to the images.

Is there a way I can have watermarked images indexed with Google?