default icon

 default icon  is an icon used to represent typically a person or author that lacks their own icon or photo, in a reply-context, or responses like comments or likes, or new accounts on silos that have not yet uploaded a photo.

Aaron Parecki
uses a gray on gray minimal abstract person icon to represent the icon of responses without one. E.g. one of the bookmarks on this post

https://aaronparecki.com/assets/images/no-profile-photo.png

GitHub
GitHub uses a blocky two color pixelated square as a default icon

Twitter
Twitter went through various default icons over the years: 

Latest as of 2017-03-31:

