var items = {n1: 'willkommen', n2: 'informationen', n3:'mediadaten', n4: 'bestellung', n5:'datenbank', n6:'newsletter', n7: 'presse', n8:'kontakt'};
var images = [];
var images_o = [];

for (var item in items) {
  images[item] = new Image();
  images[item].src = "_img/nav/" + items[item] + ".png";
  images_o[item] = new Image();
  images_o[item].src = "_img/nav/" + items[item] + "-o.png";
}

function on(item) {
  if (document.images && document.images[item])
    document.images[item].src = images_o[item].src;
}

function out(item) {
  if (document.images && document.images[item])
    document.images[item].src = images[item].src;
}