In Finder, select 'Go' from the menubar, then 'Go to Folder...'
Type in '/System/Library/Screen Savers'
This is where you System default screensavers are.
Control click on say Nature Patterns.slideSaver if thats the one you want, then 'Show Package Contents'. Then just navigate through 'Contents' folder, to the 'Resources' folder and there you will have the pictures from that screensaver.