HostPapa Knowledge Base
Scroll down to the Default Avatar section and you’ll see a few additional options are provided for selection. These designs use an algorithm to dynamically generate an avatar based on a comment author’s name or email address. Click the radio button next to a design and click Save Changes to switch the default avatar.
How to Change the Default WordPress Gravatar Image
By default, WordPress has a neat little feature that allows users to create their own personal icons identifying them when they leave a comment. They do this by signing up at gravatar.com with their email ID and WordPress checks their database whenever someone writes a comment. At the same time, many commenters would not have signed up with gravatar.com and so WordPress has the ability to assign a default gravatar to these individuals. If we go into the “Settings” section of the WordPress dashboard and select “Discussion”, we can scroll down to the “Default Avatar” area and choose what kind of image we want to leave for them.