var backImage = new Array();

  backImage[0] = "url(img/menu/menu-ppa.png) 0px 0px no-repeat";
  backImage[1] = "url(img/menu/menu-ppa.png) 0px -50px no-repeat";
  backImage[2] = "url(img/menu/menu-ppa.png) 0px -100px no-repeat";
  backImage[3] = "url(img/menu/menu-ppa.png) 0px -150px no-repeat";
  backImage[4] = "url(img/menu/menu-ppa.png) 0px -200px no-repeat";
  backImage[5] = "url(img/menu/menu-ppa.png) 0px -250px no-repeat";

function changeBGImage(whichImage) {
    document.getElementById('menu').style.background = backImage[whichImage];
}