How does Flickr prevent people from downloading images from the site?

Question

Just wondering how does Flickr prevent people from downloading images from its site? What are they using?

Answer 1

Transparent .gif over the image. You can still download the actual image by viewing the HTML source and finding the image's actual URL.

For example, a random image: http://www.flickr.com/photos/34285128@N00/4300352607/

That's the transparent image on top.

That's the actual image, which is displayed below spaceball.gif.