[flexcoders] How do I show only part of an image?

[whatabrain]

Let's say I have a 64x64 image, but I only want to display one 16x16 
section of it. How would I do this? I tried playing with 
img.transform.pixelBounds.offset(), but it didn't do anything.


<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml"; 
implements="mx.controls.listClasses.IDropInListItemRenderer">

<mx:Script>
<![CDATA[
override public function set data(value:Object):void
{
super.data = value;
img.transform.pixelBounds.offset(6,6);
}

// Implement IDropInListItemRenderer
private var m_listData:BaseListData;
public function get listData():BaseListData {return m_listData;}
public function set listData(value:BaseListData):void {m_listData = 
value;}
]]>
</mx:Script>

<mx:Image height="16" width="16" source="/images/a.gif" id="img"/>

</mx:VBox>