获得图片长宽 javascript

<img src="01.jpg" id="img1" width="100" height="20" />
<script>
var w=document.all.img1.width;
var h=document.all.img1.height;
alert("图片\n宽:"+w+"\n高:"+h)
</script>

<img src="" onload="alert(this.width);alert(this.height)">