原文出处：http://www.codefans.net/jscss/code/513.shtml
<html>
<head>
<title>等比例缩放图片</title>
<script>
function DrawImage(ImgD,iwidth,iheight){
//参数(图片,允许的宽度,允许的高度)
var image=new Image();
image.src=ImgD.src;
if(image.width>0 && image.height>0){
if(image.width/image.height>= iwidth/iheight){
if(image.width>iwidth){
ImgD.width=iwidth;
ImgD.height=(image.height*iwidth)/image.width;
}else{
ImgD.width=image.width;
ImgD.height=image.height;
}
}else{
if(image.height>iheight){
ImgD.height=iheight;
ImgD.width=(image.width*iheight)/image.height;
}else{
ImgD.width=image.width;
ImgD.height=image.height;
}
}
}
}
</script>
</head>
<body>
<img src="http://images.cnblogs.com/logo.gif" alt="自动缩放后的效果" width="220" height="80" onload="javascript:DrawImage(this,100,100)" />
</body>
</html>