ofImage and transparency

I have a few pngs that all share the same resolution and are being drawn at the same coordinates using an instance of ofImage for every image.
The images are such that they are mostly transparent and have something not transparent in different positions, so that when they are drawn on top of each other they show the intended picture.

The problem is that when I draw all the pictures one after the other I can only see the first one and the others are hidden.
What should I do to blend the alpha channels correctly?