Clear previous image

As Ned suggested - reuse the same loader. The code below works if you don't need any transitions.

import flash.display.Loader;
var XMLSource:String = "imagelist.xml";
var imgLoader:Loader;
var xmlImglist:XML;
var urlLoader:URLLoader = new URLLoader();
var urlRequest:URLRequest;
urlRequest = new URLRequest(XMLSource);
urlLoader.addEventListener(Event.COMPLETE, playlistLoaded);
urlLoader.load(urlRequest);
initLoader();
function initLoader():void
{
          imgLoader = new Loader();
          imgLoader.x = area_mc.x + 1;
          imgLoader.y = area_mc.y + 20;
          addChild(imgLoader);
}
function playlistLoaded(e:Event):void
{
          xmlImglist = new XML(urlLoader.data);
  
          for (var i:int = 0; i < xmlImglist.BgImgList.length(); i++)
          {
                    trace(i);
                    bgImageList.addItem({source: xmlImglist.BgImgList});
                    bgImageList.addEventListener(Event.CHANGE, showImage);
          }
}
function showImage(e:Event):void
{
          imgLoader.unload();
          imgLoader.load(new URLRequest(bgImageList.selectedItem.source));
}

Try this:

var imgLoader:Loader;

function showImage(e:Event):void

{

          if ( imgLoader ) {             // if was added

               removeChild(imgLoader);    // remove if was added

               imgLoader = null;        // remove from the memory old image

          }

          var imagepath:String = bgImageList.selectedItem.source;

          imgLoader = new Loader();

          addChild(imgLoader);

          imgLoader.x = area_mc.x + 1;

          imgLoader.y = area_mc.y + 20;

          imgLoader.load(new URLRequest(imagepath));

}

each time you call showImage - flash will remove old image and upload new one