flex tree 如何添加节点背景图片?

想在一级节点和子节点中加入不用的背景图片，
而不是图标或者鼠标进过颜色,弄了好久了，希望高位帮个忙
代码如下:


XML code:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
<![CDATA[
[Bindable]
private var permissionsXML:XML =
<permissions>
<permission pId="1" pName="Security Permissions" pParentID="0" checked="0" clickable="false">
<permission pId="2" pName="User Create" pParentID="1" checked="0"/>
<permission pId="3" pName="User Edit" pParentID="1" checked="0"/>
<permission pId="4" pName="User Group Create" pParentID="1" checked="0"/>
</permission>
<permission pId="11" pName="Content Permissions" pParentID="0" checked="0" clickable="false">
<permission pId="12" pName="Content Create" pParentID="11" checked="0"/>