Randomizing Header Image

On my website, thelostgeographer.net, I want to add a header image, but I also want to be able to randomize them. Certain pages already have featured images, but for the rest I want to randomize a certain set of eimages to come up.

Here is what I have so far on my header.php file.

<?php
if ( has_post_thumbnail() ) {
the_post_thumbnail(‘banner’);
}?>

Where “banner” is the featured image for those certain pages. What else do I have to add? Thank you so much for your help!