This is my version of trepmal's "Add New Default Avatar" plugin. It has a few more features which I was looking for.
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: http://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 :)
Feel free to contact me, firstname.lastname@example.org, if you have any problems or questions.