Help with removeChild or another solution

Question

I am a beginner to actionscript 3 and I've been working on a portfolio gallery in Flash that swaps XML images on click of a thumbnail.  Currently everything works well but the images stack on each other and I want them to switch on click.  I have searched all over the Internet trying to troubleshoot but the typical fix of
if(lArtLoader)
{artImageL.removeChild(lArtLoader)}
doesn't work.
Can someone please guide me in the right direction? Below is my code. Thank you so much for any help.
var artTitle:Array = new Array();
var artDescription:Array = new Array();
var artLargeImage:Array = new Array();
var artThumbnailImage:Array = new Array();
var artImageNum:Number = 0;
var maxImages:Number;
//LOAD XML
var artXMLURLLoader:URLLoader = new URLLoader();
artXMLURLLoader.load(new URLRequest("portfolio/art.xml"));
artXMLURLLoader.addEventListener(Event.COMPLETE, artProcessXML);
function artProcessXML(event:Event):void
     var artXMLData:XML = new XML(artXMLURLLoader.data);
     maxImages = artXMLData.title.length();
     for (var i:Number = 0; i < maxImages; i++)
          artTitle.push(artXMLData.title[i]);
          artDescription.push(artXMLData.description[i]);
          artLargeImage.push(artXMLData.largeImage[i]);
          artThumbnailImage.push(artXMLData.thumbImage[i]);
     loadArtThumb();
artThumbSrol.source = artThumbBox;
//LOAD THUMBNAILS
function loadArtThumb():void
     var sArtLoader:Loader = new Loader();
     sArtLoader.load(new URLRequest(artThumbnailImage[artImageNum]));
     sArtLoader.x = 137 * artImageNum;
     //STORE INFO IN THUMBNAILS
     var lArtLoader:Loader = new Loader();
     lArtLoader.load(new URLRequest(artLargeImage[artImageNum]));
     var thisTitle:String = artTitle[artImageNum];
     var thisDescription:String = artDescription[artImageNum];
     sArtLoader.contentLoaderInfo.addEventListener(Event.COMPLETE, artThumbLoaded);
     function artThumbLoaded(event:Event):void
          //ADD THUMBNAIL
          artThumbBox.addChild(sArtLoader);
          artThumbBox.buttonMode = true;
          artThumbSrol.update();
          sArtLoader.addEventListener(MouseEvent.CLICK, artLoadMainImage);
          function artLoadMainImage(event:MouseEvent):void
               //ADD LARGE IMAGE AND TEXT, SHOULD SWAP ON THUMBNAIL CLICK
               artImageL.addChild(lArtLoader);
               artHeadL.text = thisTitle;
               artTextL.htmlText = thisDescription;
               artTextL.wordWrap = true;
     artImageNum++;
     if (artImageNum < maxImages)
          loadArtThumb();

Answer 1

In this function:
function artLoadMainImage(event:MouseEvent):void
               //ADD LARGE IMAGE AND TEXT, SHOULD SWAP ON THUMBNAIL CLICK
               artImageL.addChild(lArtLoader);
               artHeadL.text = thisTitle;
               artTextL.htmlText = thisDescription;
               artTextL.wordWrap = true;
you are taking whatever is in lArtLoader and adding it to artImageL as a new child object. So with every new click you are slapping down a new image on top of whatever is already there. To get rid of any existing child objects just look at what's in artImageL and toss it out before you put in anything new. It might look something like this:
function artLoadMainImage(event:MouseEvent):void
               //ADD LARGE IMAGE AND TEXT, SHOULD SWAP ON THUMBNAIL CLICK
               if(artImageL.numChildren > 0) {
                    artImageL.removeChildAt(0);
               artImageL.addChild(lArtLoader);
               artHeadL.text = thisTitle;
               artTextL.htmlText = thisDescription;
               artTextL.wordWrap = true;