var flag=false;

 function DrawImage(ImgD, maxWidth, maxHeight){
  var image=new Image();
  var iwidth = maxWidth; //定义允许图片宽度
  var iheight = maxHeight; //定义允许图片高度
  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;
    }
    ImgD.alt=image.width+"×"+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;
    }
    ImgD.alt=image.width+"×"+image.height;
   }
  }
 }