Dinamicly changing of properties (image sourse) of movieclip

programming flash
1

Hi professionals,
I did small flash where i exported layers from Photoshop and converted them into movieclips. currently i need to change they image source to other when i click on button.
this one is required to make swf file smaller to load files after loading swf file itself.
please help me in this problem.
i found in this forum some codes and used but
i found how to do it but in this case i cant change size of picture.

function img5(e:MouseEvent):void{    
    var imageLoader:Loader = new Loader();
    var imageRequest:URLRequest = new URLRequest("images/Water lilies.jpg");
    imageLoader.load(imageRequest);
    var new_mc = t23.addChild(imageLoader);
    new_mc.x=50;
    new_mc.y=50;
}

old MC is staying there.
please help me to complete code
i need just to change picture of MC and keep all of other properties same.

Thanks in advance
Said