function getRandom(min, max)
{
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

function changeBg (){
n = getRandom (1,3);
switch (n) {
case 1: el=document.getElementById("peter"); break;
case 2: el=document.getElementById("dmitriy"); break;
case 3: el=document.getElementById("tatjana"); break;
}
el.style.display="block"; 
}