how do stack the UIImage together and wavering or swing?

Question

i will like to know how do this app stack up uiimage and wavering in UIKit.

i am following this http://stackoverflow.com/questions/13589392/xcode-using-a-uiimageview-subclass-with-nstimers to have images dropping down in random, but how do i catch it and stack the UIImage together ?

Thanks for reading and commenting.