Every X seconds
-> set bg.width to self.width+1
-> set bg.height to self.height+1
That's the simplest way to do something like that... it becomes more complicated if you want to maintain aspect ratios and stuff like that, but for simple explanation you just want to change the width and height every X seconds perhaps?
~Sol
This appears to add it and then take it away. T give like a shake kind of effect.