function resizeImage(obj, MaxW, MaxH)
{
var imageObject = obj;
     var state = imageObject.readyState;
if(state!='complete')
{
         setTimeout("resizeImage("+imageObject+","+MaxW+","+MaxH+")",50);
   return;
     }
     var oldImage = new Image();
     oldImage.src = imageObject.src;
     var dW = oldImage.width;
     var dH = oldImage.height;
     if(dW>MaxW || dH>MaxH)
{
         a = dW/MaxW; b = dH/MaxH;
         if( b>a ) a = b;
         dW = dW/a; dH = dH/a;
     }
     if(dW > 0 && dH > 0)
{
         imageObject.width = dW;
   imageObject.height = dH;
}
}

使用很简单：<img src="../pic.jpg" onload='resizeImage(this,60,90)> 就OK了