Here's a great article about the various subtexts and hidden meanings that have, over the years, been conveyed through the selection and positioning of postage stamps on postcards and envelopes.
I'm not sure I completely believe all the various claims that the article makes, but it's well-researched and well-documented and very fun to read.
The custom of the language of stamps reached different ages in different countries. In Russia, where it was a great fashion, no such postcard was published after the revolution, just as in the socialist countries after 1945. On the one hand, etiquette itself was considered a bourgeois left-over, and on the other hand the power did not tolerate any encoded message either. In western European countries, however, we find its instances as long as the end of the sixties.Woe unto those who thoughtlessly allowed their eight-year-old granddaughter to affix the stamp to the envelope, thus unintentionally sending the message: "I have discovered your deceit", when they of course meant to convey: "Many thanks for your kindness"!