I'd hope you'd look to your httpd first to handle this sort of access control. Apache's .htaccess files my fav way to keep people where they should be.

AuthType Basic
AuthName "Squid User Access Reports"
AuthUserFile /usr/local/apache/conf/passwords
<Limit GET POST>
order deny,allow
deny from none
allow from all
require valid-user 
</Limit>
[download]

Which is sloppy in some ways , this could certainly be more secure, giving your cgi carte-blanche to send users hither and thither IMHO is pleading for trouble. Doing this kind of ACL on an image by image basis ... would be pretty tricky though. Other things I've seen done , serialize your imagenames to death when they're created - so folks aren't able to guess them. Getting off topic now. bad karma. --shout down now!