Layering Components in your Flex App

ref : here

For example:

<mx:application>

   <mx:Image .... />
   <mx:Panel .... />
   <mx:Button .... />

</mx:application>

The button is on top of the panel which is on top of the image, meaning that the image is in the "background" (though not the actual background of Application as set forth by the backgroundImage attribute).