if (document.images) 
{
  n0 = new Image();
  n0.src = 'buttons/level1_a.gif';
  h0 = new Image();
  h0.src = 'buttons/level1_b.gif';
  n1 = new Image();
  n1.src = 'buttons/level2_a.gif';
  h1 = new Image();
  h1.src = 'buttons/level2_b.gif';
  n2 = new Image();
  n2.src = 'buttons/level3_a.gif';
  h2 = new Image();
  h2.src = 'buttons/level3_b.gif';

}

function Bildwechsel(Bild,Objekt)
{
  if (document.images)
    document[Bild].src = Objekt.src;
}