var onScrollHandler = function() {

  var newImageUrl = yourImageElement.src;

  var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

  if (scrollTop > 100) {

     newImageUrl = "img1.jpg"

  }

  if (scrollTop > 200) {

     newImageUrl = "img2.jpg"

  }

  if (scrollTop > 300) {

     newImageUrl = "img3.jpg"

  }

  yourImageElement.src = newImageUrl;

};

object.addEventListener ("scroll", onScrollHandler);