if (document.images) { 
  var button1_up = new Image();
  button1_up.src = "nothing.gif"; 
  var button1_over = new Image();
  button1_over.src = "Music_Note.gif";
}
 
function over_button(imgName) {
  if (document.images) {
    document[imgName].src = button1_over.src
  }
} 

function up_button(imgName) {
  if (document.images) {
    document[imgName].src = button1_up.src
  }
}