Just create many elements with the same class name like this:
<div>
<img class="mySlides" src="img_1.jpg">
<img class="mySlides" src="img_2.jpg">
<img class="mySlides" src="img_3.jpg>
</div>
And a few amendments to JS.
var slideIndex = 0;
slideshow();
function slideshow() {
var i;
var x = document.getElementsByClassName("mySlides");
for (i = 0; i < x.length; i++) {
x[i].style.display = "none";
}
slideIndex++;
if (slideIndex > x.length)
{slideIndex = 1}
x[slideIndex-1].style.display = "block";
setTimeout(slideshow, 2000); // Change image every 2 seconds
}
Hope that helps.