function image_resize( img , size ) {
  var now_width  = img.width
  var now_height = img.height
  var rate;

  if(now_width > now_height){
    rate = now_width / now_height;
    img.width  = parseInt (size * rate);
    img.height = size;
    img.style.position = "relative";
    img.style.left = "-"+ (img.width -size ) / 2 + "px";
  }
  else if (now_width < now_height) {
    rate = now_height / now_width;
    img.width  = size;
    img.height = parseInt (size * rate);
    img.style.position = "relative";
    img.style.top = "-"+ (img.height -size ) / 2 + "px";
   } else {
    img.width  = size;
    img.height = size;
  }
}