This all seems like far too much work. Just put the webpages at

http://yourserver/~youruser/hidden/SOMESECRETWORDOFTHEDAY/picture1.jpg
http://yourserver/~youruser/hidden/SOMESECRETWORDOFTHEDAY/picture2.jpg
http://yourserver/~youruser/hidden/SOMESECRETWORDOFTHEDAY/picture3.jpg
...
[download]

and then tell them to go to http://yourserver/~youruser/hidden/SOMESECRETWORDOFTHEDAY/. As long as there's an index.html at hidden (even as simple as I've got at http://www.stonehenge.com/pic/, which is my personal version of this), there'll be no listing of the contents, so no way to determine what the secret word of the day is. Change the SOMESECRETWORDOFTHEDAY frequently, and you control exactly how long someone can be out of touch with you and still see the page.

-- Randal L. Schwartz, Perl hacker