random shows wallpaper

index.js

var series = document.querySelectorAll('#series .cover');
var x = 6;
var y = 3;
var sizeX = 1200;
var sizeY = 800;
var topPos = 0;
var leftPos = 0;

var totalCovers = x*y;

var result = 0;
while (result < totalCovers) {
  var show = series[~~(Math.random()*series.length)];
  if (show.style.backgroundImage) {
    result++;
    var el = document.createElement('div');
    el.style.backgroundImage = show.style.backgroundImage;
    el.style.width = sizeX/x + 'px';
    el.style.height = sizeY/y + 'px';
    el.style.backgroundSize = 'cover';
    el.style.position = 'absolute';
    el.style.top = topPos + 'px';
    el.style.left = leftPos + 'px';
    document.body.appendChild(el);
    if (result%x === 0) {
      topPos += sizeY/y;
      leftPos = 0;
    } else {
      leftPos += sizeX/x;
    }
  }
}