if(document.images) {
  var button_a = new Image(); button_a.src = "./icons/button_a.gif";
  var button_d = new Image(); button_d.src = "./icons/button_d.gif";
  var button_h = new Image(); button_h.src = "./icons/button_h.gif";
  var button_x = new Image(); button_x.src = "./icons/button_x.gif";
}
function act(imgName, activ) {
  if (document.images){
    document[imgName].src = (activ == 1 ? button_x.src : button_a.src);
  }
}
function deact(imgName, activ) {
  if (document.images){
    document[imgName].src = (activ == 1 ? button_h.src : button_d.src);
  }
}