/*按比例生成缩略图*/
function DrawImage(ImgD,W,H){ 
  var image=new Image(); 
  image.src=ImgD.src; 
  if(image.width-W>image.height-H)
  {
    ImgD.width=W; 
    ImgD.height=image.height/(image.width/W);
  }
  else
  {
    ImgD.height=H; 
    ImgD.width=image.width/(image.height/H);
  }
}