The “Pictures folder” screensaver is quite limited – there are NO adjustments. A highly configurable slideshow screensaver in the list is GLslideshow. So a better option IMO is to configure GLslideshow to use the /home/your_username/Pictures folder and select GLslideshow as the screensaver.
The config of GLslideshow is found by running this command in a terminal window:
That brings up the screensaver config GUI for X. Locate the GLslideshow entry. Click the advanced tab. Put a tick in the “choose random image” spot. TYPE the address of your favourite Pics folder in the slot beside the Browse button.
Click the Display tab, try the Preview button. Try the Settings button too. It’s great.