var index;
 function random_num(){
    index = Math.floor(Math.random() * 4); 
}

function random_graphic(){
        graphic = new Array();
        graphic[0] = '<img src="images/photo1_promo.jpg" width="572" height="271">';
        graphic[1] = '<img src="images/photo2_promo.jpg" width="572" height="271">';
        graphic[2] = '<img src="images/photo3_promo.jpg" width="572" height="271">';
        graphic[3] = '<img src="images/photo4_promo.jpg" width="572" height="271">';
		
        document.write(graphic[index]);
}