if (document.images) {
  var back_off = new Image(); // for the inactive image
  back_off.src = "graphics/back_btn_wht_off.png";
  var back_on = new Image(); // for the active image
  back_on.src = "graphics/back_btn_wht_on.png";
}

function act() {
  if (document.images)
    document.images.backbutton.src = back_on.src;
}

function inact() {
  if (document.images)
    document.images.backbutton.src = back_off.src;
}