Self reference: edit /includes/function_user.php as follows:

'user_avatar'         => strtolower($user_row['user_email']),
 'user_avatar_type'      => 'avatar.driver.gravatar',
 'user_avatar_width'      => 60,
 'user_avatar_height'   => 60,

also edit /phpbb/avatar/driver/gravatar.php and change:

$url .= '?s=' . max($row['avatar_width'], $row['avatar_height']);

into:

$url .= '?s=' . max($row['avatar_width']."&d=identicon", $row['avatar_height']);

so users who are not registered with gravatar get an automatically generated icon istead of mystery man.