function applyBehaviour() {
  $$('.rollover').each(function(el) {
    el.onmouseover = function() { el.src = el.src.replace('.gif','_over.gif'); }
    el.onmouseout = function() { el.src = el.src.replace('_over.gif','.gif'); }
  });
  $$('.category-rollover').each(function(el) {
    el.onmouseover = function()
    {
      el.parentNode.className = 'category-thumbnail-hover';
      bits = el.href.split('/');
      $('category-image').src = '/images/categories/' + bits[bits.length - 1] + '_full.gif';
    }
    el.onmouseout = function()
    {
      el.parentNode.className = 'category-thumbnail';
    }
  });
}
Event.observe(window, 'load', applyBehaviour);