Derrick Tennant

I’d been manually adding custom default avatars in functions.php for a while, and it was always a hassle to edit it to change them. I decided to see if there was a plugin available to do the job, and stumbled upon “Add New Default Avatar” here: https://wordpress.org/extend/plugins/add-new-default-avatar/ 
It was nice, but it was missing some features such as the ability to manage avatars, upload images, and automatically resize thumbnails. So I made a few modifications 🙂
Features:
<ul>
<li>Saves current default avatar before activating and restores it when deactivated</li>
<li>Upload images using AJAX directly from the plugin page</li>
<li>Ability to remove unneeded custom default avatars from the default avatar list</li>
</ul>

Add New Default Avatar [Emrikol's Fork]

This is my version of trepmal's "Add New Default Avatar" plugin.  It has a few more features which I was looking for.

chrismoretti

Custom Default User Avatar is a fork of original <a href="https://wordpress.org/plugins/wp-user-avatar/">WP User Avatar</a> plugin. Since its original functionality was removed after a significant overhaul by the plugin author, this plugin continues the original direction from v2.2.16. This version also includes security fixes and patches as suggested by WordPress Plugins Team.

WP User Avatar 2.0

Allow users to choose custom avatars for their user accounts.

Add New Default Avatar [Emrikol's Fork] vs WP User Avatar 2.0