а почему не использовать встроенную возможность ?
Код:
setItemIcon () method
public function setItemIcon(item:Object, iconID:Class, iconID2:Class):void
Sets the associated icon for the item. Calling this method overrides the iconField
and iconFunction properties for this item if it is a leaf item. Branch items don't
use the iconField and iconFunction properties.
They use the folderOpenIcon and folderClosedIcon properties.
Parameters item:Object — Item to affect.
iconID:Class — Linkage ID for the closed (or leaf) icon.
iconID2:Class — Linkage ID for the open icon.