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 = 18;

imgArray = [
  ["images/messe/messe1.jpg","© BVBK 2009"],
  ["images/messe/messe2.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe3.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe4.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe5.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe6.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe7.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe8.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe9.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe10.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe11.jpg","© BVBK 2009"],
  ["images/messe/messe12.jpg","© BVBK 2009"],
  ["images/messe/messe13.jpg","© BVBK 2009"],
  ["images/messe/messe14.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe15.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe16.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe17.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"],
  ["images/messe/messe18.jpg","© Frank Gaudlitz / VG Bild-Kunst 2009"]
]
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];
    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];
    return false;
}