If you have what you want as a ScreenSaver you can also have it as a desktop background.
To locate the installed National Weather Screensaver :
Macintosh HD -> System -> Library -> ScreenSavers -> ScreensaverNameHere.slideSaver.
Control/right click on ScreensaverNameHere.slideSaver and select "Show Package Contents" from the contextual menu.
In the window that should appear, double-click on the "Contents" folder, then double-click on the "Resources" folder to reveal each individual photo in the ScreenSaver if that applies.
Next, select the picture you want as a desktop background and drag it to your iPhoto icon in the Dock. (or to your desktop, then onto the iPhoto icon in your Dock if that's easier)
What you have actually done here is import it into iPhoto. Once iPhoto launches and imports the pic, just select it then go to the iPhoto menu and select Share -> Desktop.
Voila ... you have a stunning new desktop picture.... you can import all the individual screensaver images into iPhoto if you like, then if you want to change your Desktop image, just do it via System Prefs.
If you choose to import all the ScreenSaver photos to iPhoto, when this is done, highlight them and from the File menu choose... "New Album From Selection". This creates a folder in the left pane of iPhoto where you can access these images easily.
When you are in System Preferences -> Desktop & ScreenSavers -> Desktop ...scroll down in the left pane until you come to the horizontal dividing line. The next folders are your iPhoto collection. Here you will find all your Albums including the newly created screensaver images folder. Just click on this folder and select the image you want to set as your new Desktop Background and that's all there is to it.
Just for your further info, there is an app (free 7 day demo then $10 to buy) called Visage. This nifty app allows you to actually run these screensavers, (complete with transitions) as a desktop background. I'm not sure how much of a drain on the CPU this would be, but you can set the CPU Usage from Low all the way through to High.
