It's really not that difficult.
Open the Finder, Navigate to System/Library/Screen Savers.
You'll see a list of the root installed screensavers from your initial OS X install.
Single click on the Forest.saver, then control-click it, or just control-click the dam thing.
On the contextual menu select "Show Package Contents".
This will open a new single pane Finder Window. From there, navigate to Contents/Resources/Images.
There you will find all of the screenshots used in the saver. option drag them out of the folder and onto your desktop, or wherever else you want to have them saved.
Then, under the Finder menu, select preferences and navigate your way to the pic you wanna use.
Also, it may seem easier to just navigate the preferences pane to the actual folder... BUT, you can't get there, the Finder doesn't show resource files in applications.
Ciao!
