Your screenshots don't have to be literal screenshots. At first glance, there doesn't seem to be anything special about your app. Its main feature is un-advertised!
I would suggest some sort of transitional photo. Show the sloppy drawing fading into the crisp flowchart. Maybe a little text on the image. "Draw flow charts in seconds!".
I second this. Use some of the frawing diagrams from the help view to show how shapes are created (I'd show at least a box and an arrow on one screenshot). It will also reduce first-use frustration trying to create shapes.
I would suggest some sort of transitional photo. Show the sloppy drawing fading into the crisp flowchart. Maybe a little text on the image. "Draw flow charts in seconds!".