gallery.js

var Gallery = function($el, path, count) {
  this.iterator = 0;
  this.count = count;
  this.$el = $el;
  this.path = path;
  this.$img = this.$el.find('js-img');
  // bind clicks
  this.$el.find('js-next-img').on('click', this.next.bind(this));
  this.$el.find('js-prev-img').on('click', this.prev.bind(this));
}

Gallery.prototype.next = function () {
  this.iterator++;
  this.show()
}

Gallery.prototype.prev = function () {
  this.iterator--;
  this.show()
}

Gallery.prototype.show = function () {
  this.iterator > this.count && (this.iterator = 0);
  this.iterator < 0 && (this.iterator = count);
  this.$img.src = this.path.replace('%num%', this.iterator);
}


// use: 
new Gallery($('.my-gallery')', images/256/255_%num%-01.png', 13);