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

chrismoretti

WP User Avatar 2.0

Allow users to choose custom avatars for their user accounts.

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>

Alternatives to Add New Default Avatar [Emrikol's Fork]

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