Applying transparency to regions of layers

Let's say I have 4 different images stacked one on top of another into 4 different layers. If I take for example the image attached to the top layer. Is there a way to apply transparency to a polygonic region of that layer, which I would describe somehow in terms of a set of coordinates, so that the image undearneath it can be seen in that region?

What I am actually trying to achieve is an image divided into polygonic regions(Voronoi diagram for example) in which the texture of each region is a piece of one of the 4 images mentioned above. I would like to achieve this using layers and transparency.

P.S. I forgot to mention the most important thing. I want to achieve this procedurally, at runtime.