This was a huge point of contention internally. So much time is spent making every pixel of every image look just so, then we're just gonna go ahead and wreck the only copy that most people will see (assuming sale to page view ratio < 50%, and nobody has the hubris to think that's not the case)?
What we settled on was a minimally intrusive watermark. In most cases, you have to play "Where's Waldo" to find it. The goal is to have the focus of the image be the image, and not have your eye drawn to a watermark then say "oh, there's a photo there too". This seems like the best balance of protecting our product without ruining our art.