window.onload = function() {
  if (!document.getElementById) return false;
  if (!document.getElementById("next")) return false;
  if (!document.getElementById("previous")) return false;
  var next = document.getElementById("next");
	var prev = document.getElementById("previous");
  next.onclick = NextImage; 
  prev.onclick = PreviousImage; 
}
var NumberOfImages = 7;

imgArray = [
  ["images/links/leider19.jpg","Porträt"," 1"],
  ["images/links/leider14.jpg","Porträt"," 2"],
  ["images/links/leider21.jpg","Fidelio, Chicago"," 3"],
  ["images/links/leider23.jpg","Isolde, Berlin"," 4"],
  ["images/links/leider27.jpg","Porträt"," 5"],
  ["images/links/leider36.jpg","Porträt"," 6"],
  ["images/links/leider46.jpg","Porträt"," 7"]
]
var imgNumber=0;

function NextImage() {
  imgNumber++;
  if (imgNumber == NumberOfImages)
    imgNumber = 0;
    document.images["startimage"].src = imgArray[imgNumber][0];
    document.images["startimage"].alt = imgArray[imgNumber][1];
    document.getElementById("imgsub").innerHTML=imgArray[imgNumber][1];
    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
    return false;
}

function PreviousImage() {
  imgNumber--;
  if (imgNumber < 0)
    imgNumber = NumberOfImages - 1;
    document.images["startimage"].src = imgArray[imgNumber][0];
    document.images["startimage"].alt = imgArray[imgNumber][1];
    document.getElementById("imgsub").innerHTML=imgArray[imgNumber][1];
    document.getElementById("imgnum").innerHTML=imgArray[imgNumber][2];
    return false;
}