Example of consistently selecting a default avatar from a set of available.

wordpress_consistent_default_avatars.php

<?php

// This is how many different "random" avatars you have.
$default_avatar_count = 20;

foreach( $comments as $comment ) {
	// Normalize the email address so user input differences doesn't change the avatar.
	// This is technically against RFC 2821, but in practice it is ok.
	$normalized_email_address = trim(strtolower($comment->comment_author_email));

	// Take the normalized email, hash it, take the most significant bits (a full sha1 would be too large an integer),
	// convert to an integer, then modulo to our count and shift to avoid 0 indexing.
	$default_avatar_id = ( hexdec(substr(sha1($normalized_email_address), 0, 7)) % $default_avatar_count ) + 1;

	// Your default avatars should be named 1.jpg, 2.jpg ...
	echo get_avatar( $comment, 32, "/avatars/{$default_avatar_id}.jpg" ); 
}