How to use local avatar images by default for new accounts/groups instead of Gravatar/Multiavatar?

In Grav I see the choice for accounts to get either Gravatar or Multiavatar avatars.

I don’t want to use either.
Instead I want to set a default avatar,


that’s ALWAYS used – unless the user is assigned to a specific group.

for e.g., if they’re a member of “Group A”, then I want their avatar to be set to


where do you set this?

Somewhere in a blueprint or other config yaml?