(function () {
  var images = ['image1.png', 'image2.gif', 'image3.jpg'];
  var step = 0;
  var timer;

  function slide() {
    document.getElementById('slide').src = images[step];

    if (step < 2)
      step += 1;
    else
      step = 0;

    timer = setTimeout(slide, 3000);
  }

  function forceSlide() {
    clearTimeout(timer);
    slide();
  }

  slide();
}());