how to popup image at the center of the desktop on mouse over

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ASP.NET

how to display image at the center of the desktop on mouse over.now i am displaying image at the bottom of the gallery but i want to display at the center of the desktop.
thanks in advance..
//this was my code in designing part

.popup
     {

         width:100px;
  height:100px;
  position:absolute;
  top:50%;
  left:50%;
  margin:-50px 0 0 -100px; /* [-(height/2)px 0 0 -(width/2)px] */
  display:none;
  top: 50%;
  left: 50%;
  margin-left: -400px;
  margin-top: -40px;

     }

function displayFullImage(imgId, url,w,h)
    {
      var pop=document.getElementById("popup");
      var img=document.getElementById(imgId);
      popup.style.display='block';
      img.style.width="550px"; //change to your image size
      img.style.height="500px"; //change to your image size
      img.src = "upload_content\\"+url;//"upload_content\\admin_Golf.jpg";//url;//
      var w = 200;
      var h = 200;
      var left = Number((screen.width/2)-(w/2));
      var tops = Number((screen.height/2)-(h/2));


    }

    function hideDiv()
    {
    var pop = document.getElementById("popup");
    pop.style.visibility = "hidden";
    }

//this function was written in master page and i am calling in content page....