js style background image by id

1document.getElementById('Demo').style.backgroundImage = "url('Test.png')";

1function bg() {
2
3   var imgCount = 3;
4   var dir = 'http://local.statamic.com/_themes/img/';
5   // I changed your random generator
6  	//floor: helps getting a random integer
7   var randomCount = (Math.floor(Math.random() * imgCount));
8   // I changed your array to the literal notation. The literal notation is preferred.
9   var images = ['001.png', '002.png', '003.png'];
10   // I changed this section to just define the style attribute the best way I know how.
11   document.getElementById('banner').setAttribute("style", "background-image: url(" + dir + images[randomCount] + ");background-repeat: no-repeat;background-size: 388px 388px");
12}
13// Don't forget to run the function instead of just defining it.
14bg();