function openWerk(wid)
{
  var url = "werk.php?werk_id="+wid;
  window.open(url, 'werk', 'top=100, left=100');
}

function changeBgImage (image, id) 
{
  var element = document.getElementById(id);
  if (image == "")
  {
    element.style.backgroundImage = 'none';
  } 
  else
  {
    element.style.backgroundImage = "url("+image+")";
  }
}