permission for downloading files

Hi all,
I'm already put into /files directory the rules:

<Files *.png>
    Order Allow,Deny
    Allow from all
</Files>

Options None
Order deny,allow
Deny from all

and with this I protect files from direct downloading but not png, to show them on pages.
But now I'm wondering which way I can control permission for downloading for Anonymous users.
I don't want to hide the files, just not permit to be downloaded if not Authenticated.
There is a way to do it in drupal? or in apache itself also

thanks in advance

luca