var imageOffArr = new Array();
var imageOnArr = new Array();

if (document.images)
{
  for (x=1; x<17; x++) {
    imageOffArr[x]=new Image();
    imageOffArr[x].src="images/website/menu/menu_a_"+x+".gif";
    imageOnArr[x]=new Image();
    imageOnArr[x].src="images/website/menu/menu_b_"+x+".gif";
  }
}

function MenuOn(num)
{
        document.images["menu"+num].src=imageOnArr[num].src
}

function MenuOff(num)
{
        document.images["menu"+num].src=imageOffArr[num].src
}