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,

user/accounts/avatars/default_avatar.svg

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

user/accounts/avatars/groupA_avatar.svg

where do you set this?

Somewhere in a blueprint or other config yaml?