@Rutra;

*"I prefer to take the risk that someone steals my images"

*You aren't taking any risk....you are inviting folks to take your images....in fact, it was just done...you have merely helped the thief.  Do you leave your doors and windows open too?

"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? "

Nothing will totally PREVENT these acts...but a watermark will MINIMIZE such acts.  It will also stop people from RESELLING that digital image or making a print from it!  These thieves are far too lazy to go to the trouble of trying to defeat that watermark....it's much easier to go steal one that doesn't have a w/m!!!!  Sure, they could post that watermarked image on their website...but at least you will get some advertising, by the time you catch them.

" 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."

Agreed!...but I will add....it is always better to try and prevent the theft in the first place. Having to go after a thief, then go to the trouble of proving ownership is being REACTIVE, not PROACTIVE.

@e-brink;
One of the concepts that I had put forward to Renderosity on several occasions is providing the option (upon upload) to have a standardized Rendo watermark applied to the image...if the artist wishes.  This is similar to what almost EVERY stock photo agency uses. 
This was rejected by Rendo as "being too difficult" to implement.

Also, the digimarc system helps...but folks can still print...and sell your images.

In closing....there are things that both artists and websites CAN do to minimize image theft...nothing is perfect, but everyone needs to do their part.

In youth, we learn....with age, we understand.