This example was shown earlier:
tell application "System Events" set ss to screen saver "Random" start ss end tell
Is there an incantation instead of "Random" to display a slideshow from the selected folder in the screen saver preferences?
On Mountain Lion (10.8)
This AppleScript will start the screen saver with the selected folder.
Lion (10.7) and earlier
First select the appropriate sources/folders by clicking the Shuffle item in the screen savers list, then checking one or more items in the dialog that appears.
Then start the screen saver as as in the Mountain Lion section, but change