Deny requests of any file without authentication

Question

Hi guys.

Try to access this image:

https://mail.google.com/mail/pimages/2/up/buzz-icon2.png

Gmail doesn't allow your request and redirect you to login, after that you will be able to see the image.

I use php + nginx + fastcgi + mongodb and can't do this trick with sessions (not with images) or http authentication (user's password must be in database), what should I do ? :(

Thanks