Chrome get height return 0

Problem:
<div id="ads_item1">
<img src="/huypv/advertise/2010-12-18-kenh14.vn.gif" />
</div>

jQuery:
$(document).ready(function(){
var menuHeight = $('ads_item1').innerHeight();
alert(menuHeight);
}

Solution:
$(window).load(function(){
var menuHeight = $('ads_item1').innerHeight();
alert(menuHeight);
}

Explain:
$(window).load => loaded image => caculate image height