I don't know anything about gnome-schedule
But my guess would be either
a) you'd remove the "while loop" and "sleep" from the script, then get gnome-schedule to run the script on you chosen schedule.
or
b) remove the script altogether, and just get gnome schedule to run:
pcmanfm -w "$(find /home/YOUR_USERNAME/Pictures -type f | shuf -n1)"
on your chosen schedule