var n=1;

function nextImage() {
   n=n+1;
   if (n<6){
     document.getElementById("preview-image").src="images/preview-" + n + ".jpg";
   } else {
     n=1;
     document.getElementById("preview-image").src="images/preview-" + n + ".jpg";

   }
}

function previousImage() {
   n=n-1;
   if (n>0){
     document.getElementById("preview-image").src="images/preview-" + n + ".jpg";
   } else {
     n=5;
     document.getElementById("preview-image").src="images/preview-" + n + ".jpg";

   }
}