function rollon(event) {
  $('img_parcours_on').src = '/images/' + Event.element(event).id + '.gif';
}

function rolloff(event) {
  $('img_parcours_on').src = '/images/parcours_vide.gif';
}

function init() {
  var areas = document.getElementsByTagName('area');
  for (i=0;i<areas.length;i++) {
    Event.observe(areas[i].id, 'mouseover', rollon);
    Event.observe(areas[i].id, 'mouseout', rolloff);
  }
}

Event.observe(window, 'load', init);