/* the image names including their extension */
images = [
 'image_1.jpg',
 'image_2.jpg',
 'image_3.jpg'
 ];

/* inwhat directory the images live */
var img_directory = 'images/';

/* the length of the images array */
var how_long = images.length;

/* the ran(dom) image function */
function ranimg(){
 if(document.getElementById("photo")){
 randomize = Math.floor((Math.random() * how_long))
 document.getElementById("photo").style.background = 'url('+ img_directory + images[randomize] +')';
 }
 }