img = new Array();
img.push('/Content/Images/fondos/fondo1.jpg');
img.push('/Content/Images/fondos/fondo2.jpg');
img.push('/Content/Images/fondos/fondo3.jpg');
img.push('/Content/Images/fondos/fondo4.jpg');
img.push('/Content/Images/fondos/fondo5.jpg');
img.push('/Content/Images/fondos/fondo6.jpg');
img.push('/Content/Images/fondos/fondo7.jpg');
img.push('/Content/Images/fondos/fondo8.jpg');
img.push('/Content/Images/fondos/fondo9.jpg');
img.push('/Content/Images/fondos/fondo10.jpg');

index = Math.floor(Math.random()*10);

window.onload = function fondo() {
    var x = document.getElementById("fondo");
    x.style.backgroundImage = 'url(' + img[index] + ')';
    }