MAC? How do I set my background as one of the screensaver pictures?

I want to change my desktop background to one of the forest, beach or cosmo pictures (which are located in the screensaver folders)



How do I find these photographs on my MacBook (Aluminium) December 2008MAC? How do I set my background as one of the screensaver pictures?
On background go to the desktop find finder (up on top of screen) %26gt; system preferences%26gt; Desktop and screen saver (located under personal heading) and select from there. You can choose to switch screen savers every 5sec or so if you want