keeping unregistered from viewing photos?

[Happy Camper]

I am trying to disallow unregistered users from viewing the large version or printable version of an image. i made the necessary changes to permissions in admin and it works if you just navigate the gallery normally but you can still:

1. access the image on the showimage.php page if you know the url
2. access the image on the printimage page if you know the url

is there a way i can prevent usergroups from ever viewing the large image even if they know the url?

thanks
eric