WP Custom Avatar – This light plugin adds the capability to change the default Avatar in your website. This is usually displayed when a person without a gravatar comments on your blog.
1. Configure the plugin by going to Appearance > Customize menu that appears in your admin menu.
2. Upload the Custom Avatar image in the WP Custom Avatar section.
3. Navigate to Settings > Discussion menu. Scroll down to the Avatars section and choose the image that you uploaded as the Default Avatar.
This plugin is released under the GPL, you can use it free of charge on your personal or commercial blog.