Okay I registered on your gallery and logged in fine
You have two admin users?
Administrator userid 1
http://www.artheon.com/gallery/member.php?uid=1
and
Admin userid 2
http://www.artheon.com/gallery/member.php?uid=2
You sure your logging with the first one Administrator as that is the one the install script would have made admin. I would suspect you created a second account called Admin and then are trying to login with that one. The Administrator one is admin of the gallery until such time as you made Admin an administrator.