Google recently demonstrated research that makes the process of identifying and removing watermarks from images automatic, which would obviously introduce a huge problem for stock agencies. Luckily, they also showed an easy solution for protecting the images.
The exploit relies on the consistency of the process: whereas a single image would prove difficult to analyze, a library of images in which the same watermark is applied in the same fashion allows a computer to quickly find what feature is consistent across all images, i.e. the watermark:
So, the exploit relies on two things: having a large library of images to analyze and that library having a consistent watermark across its images; in other words, large stock providers are most at risk. It turns out the solution is rather simple, however: by introducing small, random warping of the watermark each time it's applied, one can break the consistency the algorithm relies on, thereby causing it to leave artifacts when applied to such an image:
As you can see, the artifacts appear wherever the watermark was warped from the average (or base) version. While you likely don't need to worry about this exploit being applied to your own images, it's good to see that the research also produced more robust methods for protecting photos.