怎么把获取的屏幕宽度和高度赋值给一个图片上面

<a class="bg" href="wxdy.html"><img src="/skin/zt/wxdy/banner.gif" id="img"></a><script>

 var my=document.getElementById("img");

 var w=screen.width;

  var h=screen.height;

  document.write(w);

 my.style.width="document.write(w)+'px'";

  my.style.height="document.write(screen.height)";

//var w=document.write(screen.width);

//var h=document.write(screen.height);

  </script>

怎么值传递不对width和height那里，改怎么写。