div.swapimage {
  width: 580px;
  height: 435px;
}

div.swapimage span {
  display: block;
  width: 580px;
  height: 435px;
  background-image: url(/foswiki/pub/Allgemeines/WebHome/AlienSmall.png);
}

div.swapimage span:hover {
  background-image: url(/foswiki/pub/Allgemeines/WebHome/H2pSmall.png);
}