BuddyPress.org

So’s to not modify other files, you can also serve it up through functions.php:

//serving up a different default avatar for members
function set_default_member_avatar($url) {
return get_stylesheet_directory_uri() .’/images/new_and_improved_avatar.png’;
}
add_filter(‘bp_core_mysteryman_src’, ‘set_default_member_avatar’);

See this URL for groups avatars: https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/group-default-avatar/