array = [{src: '/files/images/elements/backsingle/back1.jpg'}, 
{src: '/files/images/elements/backsingle/back2.jpg'}, 
{src: '/files/images/elements/backsingle/back3.jpg'},
{src: '/files/images/elements/backsingle/back4.jpg'},
{src: '/files/images/elements/backsingle/back5.jpg'},
{src: '/files/images/elements/backsingle/back6.jpg'}];

function randomBG(){
var elem = document.body; // i could use the body object, but according to your css you are using the HTML element to set the background;
var bg = array[Math.floor(Math.random() * array.length)];
elem.style.backgroundImage = 'url(' + bg.src + ')';
}