var num=0;

imgArray = [
  ['laceyxbuster20dba.jpg','information', '20 Days'],
  ['laceyxbuster4.5wks.jpg','interference','4½ Weeks'],
]

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

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);
}