Brumas...here's a suggestion that you may consider.
If there was a way to 'display the answer' (before the answer was legitimately guessed correctly to your postcard puzzle) to the visitors to your friday night competition [like me for instance]...that might add to the entertainment.
It would require a little bit of computer code to allow this to happen.
perhaps...(Do you want to know the answer...but not take part).
This is what they do on Radio4's "I'm Sorry I Havn't A Clue"... when a voice says...
For those listening at home "The answer is..."
This idea would only work on known locations of postards.