screensaver that takes a screenshot
You can use ScriptSaver.


Set the AppleScript to a script containing the following:

do shell script "/usr/sbin/screencapture /path/to/folder/screensaver.png"


/path/to/folder must exist and be empty, or at least contain no other images
Set the screen saver to display after the AppleScript to Classic.
In System Preferences, set the folder for the Classic screensaver to the /path/to/folder.