Linux Mint 10 comes with xscreensaver-gl, uses GNOME screensaver daemon and does not have xscreensaver installed. Therefore, using xscreensaver-demo is not the default way to configure your screen saver and you loose a lot of configuration possibilities. As setting GLSlideshow's picture directory, timeout, with more. Luckily, it can be done easy with two files:
/etc/X11/app-defaults/XScreenSaver, here you will find
Some personal images would be nice:
Exec=/usr/lib/xscreensaver/glslideshow -root -duration 5
Here, duration is the timeout for each picture. You can find more options with:
For example fade, zoom, titles with more.
The manual for xscreensaver does also have information on the /etc/X11/app-defaults/XScreenSaver(under section X RESOURCES), but you need to install xscreensaver and might just use xscreensaver-demo to configure it then.