var num=0;

imgArray = ["images/ni.jpg","images/bh.jpg","images/nord.jpg","images/marriot.jpg","images/wr.jpg","images/canyon.jpg"]

function slideshow(slide_num) {
  document.getElementById("slide").style.backgroundImage = 'url(' + imgArray[slide_num] + ')';
}

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);
}