Hi Michael. I actually think that starting with a poster is exactly the wrong way round. Media is far more complex than that, and paper media is only one of many mediums in which messages can be transmitted. And a limited one at that. You cannot go from a poster image to a video... and to a social engagement campaign - a poster is simply an image.
Start with the principles of engagement - an idea of how you'd like to get people captivated and involved in the story... with a strong engagement idea you can work out how that translates into digital media, audio/video and then poster images - a poster image becomes easy once you've got the big picture worked out. If you start with an image as the central idea, it is too poor a concept to be able to extract video and social/digital media engagement campaigns from it. I think this has been proven many times before.