I am not a big fan of wallpapers, I usually keep it blank. But this changed On 27th Sept as my wife delivered a beautiful baby boy.
Since then my wife clicks few photos a day which I keep as wallpaper, like any new father.
There is no inbuilt option to randomize wallpapers on gnome.
Rather using any script to change wallpaper, I used below simple set of commands and scheduled a cronjob to change wallpaper every hour.
First of all set ~/wallpaper.jpg as wallpaper.
When you set wallpaper on Gnome, it gets saved in ~/.gconf/desktop/gnome/background/%gconf.xml . Now if you wish to randomize wallpaper modify %gconf.xml with desired file OR change file itself. I choose second option.
/bin/cat ~/wallpapers/`ls ~/wallpapers/ |sort -R |tail -1` > ~/wallpaper.jpg
This is all, by setting above command in cron, I get to see diff photos of my infant every hour.