function load_imgs() {
  document.img1=new Image;
  document.img1.src='bilder/top_image1_on.gif'
  document.img2=new Image;
  document.img2.src='bilder/top_image2_on.gif'
  document.img3=new Image;
  document.img3.src='bilder/top_image3_on.gif'
  document.img4=new Image;
  document.img4.src='bilder/top_image4_on.gif'
}

function swap_imgs(a, b) {
  a.src=b;
}