function changeImage(element, extensionLarge, extensionSmall) {

  var grossesBild = document.getElementById("GalleryMainImage");
  var kleinesBild = element;

  var largeImagePath = grossesBild.getAttribute('src');
  var smallImagePath = kleinesBild.getAttribute('src');
  
  //alert("Small: " + extensionSmall + "\nLarge: " + extensionLarge);
  
  var newSmallImagePath = largeImagePath.replace(new RegExp(extensionLarge, "i"), extensionSmall);
  var newLargeImagePath = smallImagePath.replace(new RegExp(extensionSmall, "i"), extensionLarge);

//  alert("Grosses Bild: " + largeImagePath + "\nKleines Bild: " + smallImagePath + "\nGrosses Bild: " + newLargeImagePath + "\nKleines Bild: " + newSmallImagePath);

  kleinesBild.setAttribute('src', newSmallImagePath);
  grossesBild.setAttribute('src', newLargeImagePath);  
}