Custom avatar question

Hello I have a premium theme which is not supported from the creator anymore so I am trying to make a change; I am not sure that this is called a bug. On comments when a visitor is place a comment; his avatar is not the default ‘mystery’ in this case (no_avatar.gif) but it automatically takes the avatar of the current post author. I like to shows the default for unregistered users. I would like to notice that the theme is use it’s own avatars. I target the code on functions but I am not able to change it, any help would be appreciated, thanks in advance here is the code:

function tgt_get_avatar_link($user_id = “”){
if (!empty($user_id)){
$avatar = get_the_author_meta(‘tgt_image’, $user_id);
}
else
$avatar = get_the_author_meta(‘tgt_image’);
if (!$avatar){
return TEMPLATE_URL . ‘/images/no_avatar.gif’;
}
return TEMPLATE_URL . $avatar;
}