var randomnumber=Math.floor(Math.random()*2);
// set bg
var underdiv = document.getElementById("under");
switch(randomnumber)
{
    case 0, 1:
        underdiv.style.background = "url('images/syd.jpg')";
        break;
    case 1:
        underdiv.style.background = "url('images/hk.jpg')";
        break;
}