no it uses cpu and gpu only when they render from my understanding.
what i said above is that imagine 10 instances of your enemy all going left and right with your cellphone trying to animate 200 png's per second for each!
so although when they don't animate you don't use processing power you will face another problem with many different png's.
because mobiles at the moment load all assets from all your layouts at startup the cellphone does not have enough ram and it either crashes or it exits.
there is yet no layout by layout loading.
so the more png's the more stress you put on your cellphone when the game starts for the first time.