var img_names = ['startseite', 'europa_tanzt', 'ballamasch_2006', 'service', 'events', 'medien'];

var img_off  = new Array();
var img_over = new Array();

for (var i in img_names) {
  id = img_names[i];
  img_off[id] = new Image();
  img_off[id].src = "_img/nav/" + id + ".jpg";
  img_over[id] = new Image();
  img_over[id].src =  "_img/nav/" + id + "-over.jpg";
}

function on (id) {
  if (document.images && (img = document.images["i_" + id]))
    img.src = img_over[id].src;
}

function off (id) {
  if (document.images && (img = document.images["i_" + id]))
    img.src = img_off[id].src;
}