pics = new Array();

 pics[1]="../gifs/menu/photo01.jpg"
 pics[2]="../gifs/menu/photo02.jpg"
 pics[3]="../gifs/menu/photo03.jpg"

 rnd = (Math.round((Math.random()*2)+1))

function changeBgImg() { 
document.getElementById('menu').style.backgroundImage = 'url('+pics[rnd]+')';}