Can't unload a Loader =(

I can't seem to be able to figure out how to unload a JPG image once I've loaded it so I can load a second JPG in it's place. Here's my code so far (document class, picHolder_mc exists already on the timeline). All that happens is the second JPG being called loads on top of the first. Any help greatly appreciated.

import flash.display.*;
import flash.events.*;
import flash.net.*

public var pdfA_Loader:Loader = new Loader();

public var pdfA_URL:URLRequest;

public var picHolder_mc:MovieClip;      // mc on timeline

public function loadJPG():void {

     pdfA_Loader.unload();
     pdfA_URL = new URLRequest("myJPG.jpg");
     pdfA_Loader.load(pdfA_URL);
     picHolder_mc.addChild(pdfA_Loader)    

}