var num=0;

imgArray = [
  ['img/midsouth1.jpg'],
  ['img/midsouth2.jpg'],
  ['img/midsouth3.jpg'],
  ['img/midsouth4.jpg'],
  ['img/midsouth5.jpg'],
  ['img/midsouth6.jpg']
]

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);
}