function Wechsel (bildurl) {
  document.getElementById("foto").src = bildurl;
  document.getElementById("aktiveUrl").href = bildurl;
  document.getElementById("aktiveUrlDiv").setAttribute("value",bildurl)
  var arr = document.getElementsByClassName("nav_img");
  for (x = 0; x < arr.length; x++)
  {
    if (arr[x].firstChild.firstChild.href.indexOf(bildurl) != -1)
    {
      arr[foundIndex].firstChild.id = "nav_img_active";
    }
  }
}