The postcards show the image of Ignudo 5, the head of the Ignudo at the upper right corner of the Drunkenness of Noah (above the Delphic Sibyl).
The Ignudi (singular: ignudo, from the Italian adjective nudo, meaning "naked") are the 20 athletic, nude male figures that Michelangelo painted at the four corners of the five smaller scenes of Creation. Because they were not relevant to the themes of the piece, Michelangelo's ignudi outraged several pontiffs.
Most of the figures are surrounded by a huge garland of oak leaves, and clustered about them are thousands of acorns resembling the penis, or "prickhead", in Tuscan slang (testa di cazzo). The most likely reason for their abundance is that Pope Julius II, who commissioned the work, was of the della Rovere family: they function as, perhaps, Michelangelo's allusion to his patron.
Sistine Chapel, The Ignudi