One of the misconceptions about watermarks is that watermarks prevent image piracy. If someone (any teenager with some spare time and reasonable Photoshop skills) wishes to misappropriate an image there are many ways to subvert a watermark, beginning with simply cropping it out. Also, anyone with the same skills knows that even if right-click is disabled, they simply need to take a screen shot to capture the image and crop out the rest.
We never display images which can reasonably be printed. All of our web previews are limited to a maximum dimension of 650 pixels - hardly print worthy - at 300 dpi this would render a print of a little over 2" x 2". Likely you know that you can log into one of the most valuable image collections in the world - Corbis - and view non-watermarked images once you open your free account. The images, like ours, are low-res. However, anyone who has advanced understanding of image manipulation could use a tool like Genuine Fractals to interpolate the image UP in size and print a larger image. It is never as good as the original file but if you are stealing you probably don't mind letting Genuine Fractals do it's job as you aren't paying for quality anyway. The same can be said for Getty...the largest stock photo house in the world. These too can be viewed without watermarks.
The issue, like most things in life, is a trade off. Either artists give buyers a decent view of the object which we are asking them to purchase and risk a small percentage of people who know how to manipulate it upwards OR we can watermark all images in all cases and ask buyers to look through the watermarks to make their purchase decision.
Because of the above, not every photographer will sell images through Corbis and Getty in spite of their obvious size and distribution abilities. A great deal more professionals, however, try very hard to get this distribution and many are not selected. There is no easy answer, but Getty and Corbis' situation is exactly the same situation in which we find ourselves. We have decided to provide a high quality buying experience as if most buyers are honest and legitimate shoppers. We believe it is the best way to run a business of this kind. Yet, as we went into the business we knew that 1 in 50 artists would not agree with that perspective and as a result, we offer watermarking as an option for artists who wish to protect their images regardless of size or resolution.
It's the challenge we face living in a world of gray vs. black and white! Believe it or not, we received email from an artist who said he would NEVER sell on Imagekind because we do not allow large enough previews to "show the full beauty of the work."