For part of an app I have what I think should be an easy task but I don't even know where to start. I need to bring up two images and based on where the user is touching create a mask through to reveal the image underneath. If you know of a direction to point me that would be appreciated.

A very simple solution is to setup a set of buttons that have no background so the user does not see them.
Each button then when tapped causes the relevant texture to be rendered.

You can do this with two UIImageView controls and CoreAnimation effects.