[scikit-image] Outer points detection

Hi everybody, I am working on a computer vision problem but I got stuck. I am trying to detect outer points of an binary image (see attached images, points which i need are labeled in red on image points.png). I have tried different approaches (edges, maximas, corner detection) but usually the inner corners will be detected because the outer points I need are rather round. Any idea of an algorithm which might be useful for this problem? Thanks a lot in advance, Stefanie