How can I add an overlay image to staff/group profile pictures?

[cpradio]

I think you are looking for this

Automatically add group flair to members' avatars

Overview Group avatar flair displays to the bottom-right of a user’s avatar, providing a stylish way to highlight a user’s group membership. When creating or configuring a group, an admin can choose an image or Font Awesome icon to use as group flair. Below are the currently supported contexts, using our logo as an example: User cards [usercard] Topic/PM posts [posts] Topic maps [topic-summary] User profile banners [profile] User summary “Most…” lists [most] Users…