Niketalk, how do I do this image editing trick? Vol layers, transparency, collage

I want to simulate the first one almost entirely, although probably I'll keep the hands and background normal and put the image collage inside the shape made. All I know is that it involves layers. I would need to somehow select the portion of the real image I don't want and turn it transparent, I guess delete it if that layer has a transparent background? I don't even know where to begin. If I knew what it as called even, maybe I could look up some tutorials. I don't have Photoshop but I have GIMP and Colar on Mac which provide basically the same functionality in freeware.