var backImage = new Array();

backImage[0] = 'images/1.jpg';
backImage[1] = 'images/2.jpg';
backImage[2] = 'images/3.jpg';
backImage[3] = 'images/4.jpg';
backImage[4] = 'images/5.jpg';


    function changeDivImage(whichimage)
    {
        var imgPath = new String();
        imgPath = backImage[whichimage];
            document.getElementById("overlay").style.backgroundImage = "url("+imgPath+")";
     
    }