$(document).ready(init);

function init()
{
  $('#header-menu img').each(function()
  {
    this.srcOrig = this.src;
    this.srcOver = this.src.replace('/menu/', '/menu-over/');
    var img = new Image();
    img.src = this.srcOver; 
  }).mouseover(function()
  {
    this.src = this.srcOver;
  }).mouseout(function()
  {
    this.src = this.srcOrig;
  });
  
}