var randomnumber=Math.floor(Math.random()*5);
randomnumber = randomnumber+1;

switch (randomnumber)
{
case 1:
  bgimg = 'https://lh5.googleusercontent.com/-f_20mlyNkYQ/TrrE4qu8a-I/AAAAAAAAXXY/C5blySyGVS0/s0/header1.jpg';
  break;
case 2:
  bgimg = 'https://lh3.googleusercontent.com/-Xw7U7lqec7c/TrrE4TYrdSI/AAAAAAAAXXU/Ruvdq4ywz7o/s0/header2.jpg';
  break;
case 3:
  bgimg = 'https://lh5.googleusercontent.com/-rCay0rCf0UY/TrrE5A-atKI/AAAAAAAAXXc/QhAcS_EsDHQ/s0/header3.jpg';
  break;
case 4:
  bgimg = 'https://lh5.googleusercontent.com/-wtPEJxUuEdU/TrrE5lboZ0I/AAAAAAAAXX0/6YeOwlOUOG8/s0/header4.jpg';
  break;
default:
  bgimg = 'https://lh5.googleusercontent.com/-mOn9y5CLeYQ/TrrE52MvvzI/AAAAAAAAXXw/ieOrQHAfL84/s0/header5.jpg';
}
document.write('<style>#wrapper .header{background-image:url('+bgimg+');</style>');