// implement in State class
private function addParallaxFill(paralax:Bitmap, tmxWidth:Number, tmxHeight:Number):void {
	var px:Number = (paralax.width - stage.stageWidth) / (tmxWidth - stage.stageWidth);
	var py:Number = (paralax.height - stage.stageHeight) / (tmxHeight - stage.stageHeight);
	var co:CitrusSprite = new CitrusSprite("parallax", {parallaxX: px, parallaxY: py} );
	co.view = paralax;
	add(co);
}

useful function for Citrus Engine

CE: add parallax fill image

ActionScript 3