css如何自动控制图片大小？css实现自动调整图片大小

//  方法：setSelectreadonly  用于设定极select控件ReadOnly,
//这个一个模拟只读不是真的只读
//使用了onbeforeactivate,onfocus,onmouseover,onmouseout事件
//示例：< img src='img.jpg' onload='ImgAutoSize(ImgD,FitWidth,FitHeight)' > ;
//  create by sl 
// ---------------------------------------------------
function ImgAutoSize(imgD,FitWidth,FitHeight) 
{
var image1=new Image(); 
image1.onload = function ()
{
if(this.width>0 && this.height>0) 
{ 
if(this.width/this.height>= FitWidth/FitHeight) 
{ 
if(this.width>FitWidth) 
{ 
imgD.width=FitWidth; 
imgD.height=(this.height*FitWidth)/this.width; 
} 
else 
{ 
imgD.width=this.width; 
imgD.height=this.height; 
} 
} 
else 
{ 
if(this.height>FitHeight) 
{ 
imgD.height=FitHeight; 
imgD.width=(this.width*FitHeight)/this.height; 
} 
else 
{ 
imgD.width=this.width; 
imgD.height=this.height; 
} 
} 
}
image1 = null;
}
image1.src=imgD.src; 
imgD.style.cursor = 'hand';
imgD.onclick= function(){openWin(this.src,'imgphoto',600,400)};
imgD.title = "点击在新窗口中查看原图";
}