It adds the available to screen saver attribute to that picture.
When the pluto screen saver tool scans for pictures it only accepts pictures with that attribute set. And then displays them (semi) randomly fading in and out and panning around.
But if you still have your Flickr script running, or at least pictures downloaded by Flickr when it was running, then you will have to keep a sharp eye open as they will be mixed in among "randoms"! Just use the admin site to delete the flickr pictures after you have disabled the script.