function mouseOverProc()
{
  var s = this.src;
  this.src = s.replace(/^(.*)\.gif$/, "$1_a.gif");
}
function mouseOutProc()
{
  var s = this.src;
  this.src = s.replace(/^(.*)\_a.gif$/, "$1.gif");
}
function init()
{
  var im = document.getElementsByTagName('img');

  for (var i = 0; i < im.length; i++){
    var t = im[i];
    if (t.className == "hover"){
      t.onmouseover = mouseOverProc;
      t.onmouseout = mouseOutProc;
    }
  }
}

window.onload = init;