Masking one image through to another

[poomah]

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.

 

[iPadTester]

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.

 

[MichaelBranton]

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