Does not work anymore but there is another Script with the same function around. Found via google: http://http.ch/background-picture-from-national-geographic-daily-automatic-update

Does what it says. Nice and simple. I missed this since a widget that did it (what is now the Yahoo Widget) was stopped in development.

Still, I would like to have it place the photos in a subfolder of Photos, so things don't get too messy. I may look at the script myself or if the author will update it...