Disk1=new Image();
  Disk1.src="ueberweiss.gif";
  Disk2=new Image();
  Disk2.src="ueberblick.jpg";
  

function wechsel1()
{
 window.document.images[0].src=Disk2.src; 
  }

function wechsel2()
{
 window.document.images[0].src=Disk1.src;
}