var num=0;

imgArray = [
  ['images/Investigations/Invest20090815_01.JPG','Group Photograph','1/3 - Group Photograph'],
  ['images/Investigations/Invest20090815_02.JPG','Group Photograph','2/3 - Light Anomaly'],
  ['images/Investigations/Invest20090815_03.JPG','Light Anomaly','3/3 - Light Anomaly']
]

function slideshow(slide_num) {
  document.getElementById('mypic').src=imgArray[slide_num][0];
  document.getElementById('mypic').alt=imgArray[slide_num][1];
  document.getElementById('burns').innerHTML=imgArray[slide_num][2];
}

function slideshowUp() {
  num++;
  num = num % imgArray.length;
  slideshow(num);
}

function slideshowBack() {
  num--;
  if (num < 0) {num=imgArray.length-1;}
  num = num % imgArray.length;
  slideshow(num);
}