//参数(图片,允许的宽度,允许的高度)
function DrawImage(ImgD,iwidth,iheight)
{
  var image=new Image();   
  image.src=ImgD.src;   
  if(image.width>0 && image.height>0)
  {   
    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;   
      }   
    }   
  }   
}