This bug has been filled here:

https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/145152

"I noticed that the "Pictures folder" screensaver shows all pictures out of $HOME instead of only $HOME/Pictures. Even pictures with sensible information out of my projects or pictures out of the cache of my web browser - ~/.mozilla/firefox/<random>.default/Cache - will be displayed. This might lead into showing pictures which you don't want to display in public.

I've got a folder called "Pictures" inside my home dir.

# ls -al ~/Pictures/
insgesamt 880
drwxr-xr-x 2 toff toff 120 2007-09-26 11:53 .
drwxr-xr-x 102 toff toff 4168 2007-09-26 12:00 ..
-rw-r--r-- 1 toff toff 252369 1999-11-20 00:00 1.jpg
-rw-r--r-- 1 toff toff 248626 1999-11-20 00:00 2.jpg
-rw-r--r-- 1 toff toff 390383 1999-11-26 00:00 3.jpg

But these pictures don't get displayed. Instead of those three pictures, gnome-screensaver looks in my whole $HOME for pictures.

This is not a real security threat. But since there is a option for a random screensaver. Users might find this "feature" quite unpleasent, when co-workers find out what the boss is doing late at night only by watching the screensaver..."