Mixing faces of two persons

I have two images that I want to split each into 3 parts. If I click on the开发者_开发技巧 first image, the first part for eg forehead then second image first part i.e forehead will come by replacing first part and remaining two parts of the first image should be unaltered. Likewise all three parts should change while clicking that part how can I achieve this?

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Why not just create a set of UIButton objects stacked on top of eachother with user interaction enabled and set the images in those.. then when one button is tapped, you check which one was tapped based on sender or tag and hide the button pressed, but show the next one underneath it? When you get to the last button in the stack, show the top button again.

This would pretty much be a switch-case just setting hidden to yes/no.