Tile List Selection indicator

Question

I have a tile list layout with some thumbnails.
When one is selected it gets a default blueish background.
Could someone explain how i would go about putting a nice green tick over a thumbnail once its selected?

Answer 1

Thank you both for your replies.
I'm already using a custom component that extends a spark List with layout property set to TileLayout. This is so i can mimick the user holding the CTRL key for multiple selection.
I also already have a custom item renderer that displays the thumbnail and fires off an event when clicked.
Really confused as where to put either of your code snippets.
this is my custom component
package
     import flash.events.MouseEvent;
     import mx.core.IVisualElement;
     import spark.components.List;
     public class CheckList extends List
          public function CheckList()
               super();
               allowMultipleSelection = true;
           * Override the mouseDown handler to act as though the Ctrl key is always down
          override protected function item_mouseDownHandler(event:MouseEvent):void
               var newIndex:Number = dataGroup.getElementIndex(event.currentTarget as IVisualElement);
               // always assume the Ctrl key is pressed by setting the third parameter of
               // calculateSelectedIndices() to true
               selectedIndices = calculateSelectedIndices(newIndex, event.shiftKey, true);
and this is my item renderer
<?xml version="1.0" encoding="utf-8"?>
<s:ItemRenderer xmlns:fx="http://ns.adobe.com/mxml/2009"
                    xmlns:s="library://ns.adobe.com/flex/spark"
                    xmlns:mx="library://ns.adobe.com/flex/mx"
                    autoDrawBackground="true" width="300" height="315" click="thumb_clickHandler(event)">
     <mx:Image id="thumb" x="25" y="26" width="250" height="265" source="{data.thumburl}"/>
     <fx:Script>
          <![CDATA[
               import flashx.textLayout.factory.TruncationOptions;
               import mx.controls.Alert;
               import mx.events.ItemClickEvent;
               protected function thumb_clickHandler(event:MouseEvent):void
                    var ev:ItemClickEvent = new ItemClickEvent(ItemClickEvent.ITEM_CLICK, true);
                    ev.item = data;
                    ev.index = itemIndex;
                    //Alert.show(ev.item.imageurl);
                    dispatchEvent(ev);
          ]]>
     </fx:Script>
</s:ItemRenderer>
Any help you can give me would be great, the code is getting really messy, should i merge these into one class?