Still obsessed with this, I did some internet sleuthing. Straight from the creators mouth:
"Any stories you'd like to share about making these?
I'm not sure it's a story, but a lot of folks have been puzzled by the Wizard of Oz poster. The last symbol in particular. And with the recent boom in the popularity of the series this trend has become more apparent.
Since I don't consider myself an artist, I don't care about the mystery or letting the audience interpret the work for themselves, so I'm just going to tell you. Some have guessed it, some are denying it, but most everyone is thinking it. They're balls. Big… hairy… balls.
In the story, the Cowardly Lion was the only one who requested something intangible. So, I had to make it tangible. Coincidentally, it worked out in the overall balance of the design to use an upside-down heart. It still makes me laugh."
I'll also cite my source http://www.mymodernmet.com/profiles/blogs/minimalist-fairy-tale-posters