var plus = new Image().src
plus = "images/plus.gif";
var minus = new Image().src
minus = "images/minus.gif";

function changeMenu(id)
{
  var img = document.getElementById(id);
  var sub = document.getElementById(id + "_sub");
  if(sub.className == "on")
  {
    sub.className = "off";
    img.src = plus;
  }
  else
  {
    sub.className = "on";
    img.src = minus;
  }
}