take a look in the code archives. I think there's a space invaders clone or two out there. Look at other people's code and it should give you lots of insight into how to get rid of that flicker.
As to the sizes of the images... There's no "correct" sizes. Anything you want to put on the screen is fine with blitz.
As to the sizes of the images... There's no "correct" sizes. Anything you want to put on the screen is fine with blitz.