Maintain image aspect ratio

Hi, i try to charge a XML images inside a movieclip, but i need this movie clip maintain the aspect ratio. This movie clip gonna change with the size of the explorer, but when this change my image is going stretch. I used this code doesn’t work but i try:

stageListener.onResize = function() {
	MC._x = Math.round((Stage.width/2)-(MC._width/2));
	MC._y = 153;
	MC._height = Math.round(Stage.height-200); 
};

Stage.addListener(stageListener);

My images have a size of W=900px by H=600px

and sorry for my bad english :beam: