What Kimberley said is right, but you can also do it with sprites. You just have to do it a little differently. Your sprites show up in the order that you create them, with the ones that you create first in the back, and the ones that you create last in the front. When you call your
crazyBackground functions, you are creating new sprites, so they are going to show up on top of everything else. What you should do instead is create a single background sprite at the top of your code, before you create any other sprites. Then, in your function, only set the background sprite's animation to the image you want, rather than making an entirely new sprite.