buttonImg = new Array();
for (i=0; i<=2; i++){
  buttonImg[i] = new Image();
  buttonImg[i].src = "btns/btn" + i + "_on.gif";
}
function imageOn(n){
  document.images['menu'+n].src = buttonImg[n].src
}
function imageOff(n){
  document.images['menu'+n].src = "btns/btn" + n + "_off.gif";
}