function DrawImage(ImgD,iwidth,iheight){
var flag=false;
 var image=new Image();
// var iwidth = 150;
// var iheight = 100; //定义允许高度，当宽度大于这个值时等比例缩小
 image.src=ImgD.src;
 if(image.width>0 && image.height>0){
  flag=true;
  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;
   }
  }
 }
} 
//调用：<img src="图片" onload="javascript:DrawImage(this,100,100)">