用javascript 怎么判断图片是否加载完成

<script language="JavaScript">

function showImage(url){
var o = document.createElement('img');
o.src = url;
document.body.appendChild(o);
}
function loadImage(url){
var o= new Image();
o.src = url;
if(o.complete){
showImage(url);
}else{
o.onload = function(){
showImage(url);
};
o.onerror = function(){
window.alert('图片加载失败:'+url);
};
}
}

loadImage("http://img02.banzhu.net/User_UploadFiles/2008/6/18/20086180001876457023.jpg");

</script>