Here's a novel concept. Or, perhaps it's not novel, but it is the first example of this I have in my collection.
From top to bottom, the first two images show the front and back of the card, with the image folded over the sender's message; the second two images show the front and back with the flap opened up, creating a larger visual on the front, and revealing the "private" message on the back.
Actually, the cards are called Fold 'n Please cards, from a company called Nouvelles Images. On their website in the history section, they assert that Jacques Blanc (founder?) foresaw the growth of interest in art, and in 1957 was the first to ask artists for permission to put their work on postcards.
That's definitely nouvelle!