change css picture with javascript

Question

So i try to random my background every second but my background cant fix with my website so i need to using css for doing that ( cant do that with this code ) . So can we using css in javascrpit ? this is my code

function changeBg() {
    var images = [],
    index = 0;
    images[0] = "0.jpg";
    images[1] = "1.jpg";
    images[2] = "2.jpg";
    index = Math.floor(Math.random() * images.length);
    document.body.style.background ="#fff url('"+images[index]+"') no-repeat center center fixed";
}

setInterval(changeBg, 3000)

so i want all picture in this js to be using this css . how can i do that . Thank you guys so much :

 -webkit-background-size: cover;

  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;