<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/03/displaying-an-error-tool-tip-for-an-image-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Image id="img"
            source="http://blog.minidx.com/images/404.gif"
            width="160"
            height="120"
            ioError="img.errorString = event.text;" />

</mx:Application>