How to Add a Random Custom Gravatar in the WordPress Comments?

Have a look at the Codex about “Using Gravatars”. There you´ll find a part about “Checking for the Existence of a Gravatar” which works like this:

The trick to do this is to specify “404” as the default. In this case, the gravatar service will return a 404 error if no gravatar exists, instead of returning some default image. A real image will get a 200 code. It is best to check for 200, as some other errors might be returned as well, for other cases.

There is also a code snippet you can use for that check.

Then build something like this for your default avatars in single.php/comments.php or whatever you are using:

$default_avatars = array(
                      'yoururl.com/whatever/static/ava1.jpg',
                      'yoururl.com/whatever/static/ava2.jpg',
                      'yoururl.com/whatever/static/ava3.jpg',
                      'yoururl.com/whatever/static/ava4.jpg',
                      'yoururl.com/whatever/static/ava5.jpg'
                   );
$my_default_avatar_now = array_rand($default_avatars);

echo '<img src="'.$my_default_avatar_now.'" class="avatar" />';

array_rand() seems to not have the best randomness from what you read in the comments on php.net but maybe it´s working fine for you.

This code is untested, so it´s more an explanation of the way you could go then a ready-to-use solution.