Picturethrill is a tiny app which will automatically download a new image for your desktop wallpaper every day.
Launch the program and it'll download and display thumbnails of the current images from every supported site: Bing, Nasa, Earth Science, National Geographic and Nature. All you have to do is choose the site you'd like to support, click Wallpaper, and it'll download the full-sized image, set it as your wallpaper, then update the image every day.
Picturethrill is surprisingly simple. It doesn't add a system tray icon, for instance - there's no extra component running permanently in the background (the program is launched and does its work at boot time).
And there are very few settings. You can prevent Picturethrill updating your wallpaper daily, and tell it just to collect and save new images for you, but that's about it.
You may well find you'd like a few more configuration options. What if you didn't want to choose just one source site for your pictures, for instance? Or maybe you'd prefer to change your wallpaper at some frequency other than every 24 hours? Then you're out of luck - there's no such configurability here.
The lack of tweaks does make for a simple and lightweight tool, though, and if that's your priority then Picturethrill could be a very good choice.