// define a global object for the header sizes
var headerobject = new Object();

headerobject.hideBackground = function()
{
  document.getElementById('header_background').getElementsByTagName('img')[0].style.display = 'none';
}

headerobject.showMenu = function()
{
  document.getElementById('mainmenu').style.display = 'block';
  document.getElementById('header_background').getElementsByTagName('img')[0].style.display = 'block';
}