if (document.images) {

  roll1on = new Image();
  roll1on.src = "_img/btn_h_donate-on.gif";

  roll1off = new Image();
  roll1off.src = "_img/btn_h_donate-off.gif";
  
}

function changeImages() {
  if (document.images) {
    for (var i=0; i<changeImages.arguments.length; i+=2) {
      document[changeImages.arguments[i]].src = eval(changeImages.arguments[i+1] + ".src");
    }
  }
}