Dont get me wrong, I perfectly understand artists who watermark their images but, personally, I just hate seeing images with visible watermarking. Imagine "Creation of Adam" with a watermark, imagine Escher or Dali images with watermarks. For me, visible watermarks completely ruin my enjoyment of watching a beautiful image. Absolutely, completely, totally.
I know this is an emotional judgement, not a rational one, but watching art is a deeply emotional experience, not a rational one.

I do not put watermarks in my images. I even try to make my signature as small as possible. I prefer to take the risk that someone steals my images. This has happened at least twice, that I know about. I dont like it but I prefer this to the alternative of putting visible watermarks.

Anyway, a visible watermark doesnt really prevent acts like the one being discussed in this thread. If someone wants to post an image they dont own in their site, it means that these people are not too concerned about copyright violation and so, why would a watermark stop them from doing it? I think the main issue is about proving ownership of an image and for that it's not necessary visible watermarking, an invisible embedded digital ID serves this purpose a lot better and doesnt "steal" the enjoyment of watching a beautiful image.