ItemRenderer display image according variable

Question

I have the following code from an InterRenderer, where I try to display two kind of icons depending on the value of its labelField within a Datagrid.

<s:GridItemRenderer xmlns:fx="http://ns.adobe.com/mxml/2009" 
            xmlns:s="library://ns.adobe.com/flex/spark" 
            xmlns:mx="library://ns.adobe.com/flex/mx">

<fx:Script>
    <![CDATA[
        import mx.controls.Alert;
        import mx.controls.dataGridClasses.DataGridListData;
        import mx.controls.listClasses.BaseListData;

        import spark.components.DataGrid;

        private var _listData:BaseListData;
        [Bindable]
        private var _isSelected:Boolean;

        override public function prepare(hasBeenRecycled:Boolean):void {}

        override public function set data( value:Object ) : void  {
            super.data  = value;
            //Alert.show( value.sent );
            _isSelected = data.sent=='1'?true:false;

        }

        [Bindable]public function get listData() : BaseListData {
            return _listData;
        }
        public function set listData( value:BaseListData ) : void {
            _listData = value;
        }

    ]]>
</fx:Script>

<s:Image id="btn_ok" horizontalCenter="0" source="@Embed('assets/images/ok_icon.png')"
         visible="{_isSelected?true:false}"
         verticalCenter="0"/>

<s:Image id="btn_ko" horizontalCenter="0" source="@Embed('assets/images/send_icon.png')"
         visible="{_isSelected?false:true}"
         verticalCenter="0"/>

The question is that I need to link the value of the labelField and in case it changes, one image hides and the other shows.

What's wrong in my code?

Thanks.