Okay so I did some reading on Flickr. Stupid I never really bothered to check this before. The default licence for images uploaded is All rights reserved, meaning the image may not be used in any way without permission from the photographer, as I understand it.
I do make it a practice to only upload small-resolution images, but I don't think I'd want to put in a watermark. It draws attention and changes the image in a way that I think is not worth lowering the risk of the image being used by others only slightly.
On a different note, what would be the stept one could take if someone else used one of your images? The case of the guy in the video is incredibly severe of course, there's no way he's going to keep this under control any more. However, do any of you have any stories or experiences where someone used your image without permission, and how did you deal with it?
I have this rather funny story that is somewhat related: me and some friends have a website where we occasionally put up general interest articles. Then some day one of us stumbles upon another website that literally copy-pasted the article and did not mention us as the authors of it. He also didn't ask for permission before copying it. Since the website was based in our own country we sent the owner a serious letter forcing him to take the article off his website, otherwise we'd contact a lawyer. In the meantime however, we discovered he'd not copied the images in the article to his own server, but instead he was still linking to the images on our server. So we changed the files that were located in the folder he was linking to, to..ehm...well..different images. It looked rather funny :)