如何得到一张图片的长与宽是多少像素

1<img onload="alert(this.offsetHeight);alert(this.offsetWidth)" src="abc.gif"/>

1<img height="29" id="guagua" onclick="aa()" src="open.gif" width="39"/>

1<script language="JavaScript">   
2function aa()   
3{   
4b=document.getElementById("guagua");   
5document.write(b.width);   
6document.write(b.height);   
7}   
8</script>

1<img border="0" onload="alert('filesize : '+ this.fileSize +' Byte\r\nwidth : '+ this.clientWidth + '\r\nheight : '+ this.clientHeight)" src="http://expert.csdn.net/images/csdn.gif"/>