It looks like a poster but was drawed by hand in the waiting room of a Swiss clinic for children. I didn't do it myself, my father painted it. The secret is quite easy, what you need to have is a projector and it has to be dark in the room. This allows you to draw all the lines and have the perfect proportions. But it may sound easier than it is, you need to have a skilled hand for a result like this. But try it ;)