Well a cheapt solution could be, use events:
System-> Every 11 seconds -> FirstBackground->changeopacity->0
System-> Every 23 seconds -> SecondBackground->changeopacity->0
System-> Every 37 seconds -> ThirdBackground->changeopacity->0, FirstBackground->opacity->100,SecondBackground->opacity->100.
Something like that could work, using opacity and three background, make use of the Z-order so when you change the opacity in the first background, the second one is showed