You could use the auto generated Gravatar options in the back end which will randomly assign an “avatar” to those who do not have one. You will find this when you go to your WordPress back-end Settings > Discussion and scroll down to the bottom.
Or you can have a custom avatar replacement for those who do not have one however this will not be random but will ad a custom a default avatar to your site. You can do this by simply putting this in your comments.php file via your theme where you would be putting your avatars:
<?php echo get_avatar( $comment, 96, get_bloginfo('template_directory') . '/images/avatar.jpg' ); ?>
Just make sure you have an image in your images folder called avatar.jpg . By the way 96 is the size of the avatar in pixels. You can adjust the size by making the number larger (for a larger avatar) or smaller.
I hope this helps. 🙂
EDIT:
I am not sure if this is something you might be interested because it allows people to upload their own avatars and is a Gravatar alternative. However this plugin does say that it allows avatar control. I have not tried it but you might want to give this a go: