Image Clipping Mask

Hi all!

I’m learning Axure at the mo and I can’t work out how to create a ‘clipping mask’ - Something very useful in other softwares. I’m trying to achieve the effect shown in the attached, whereby a click changes the size of the mask. I have shown the two moving shapes in the attached slightly differently so you can see easily what is happening.

There is some delay in the prototype click action working. Please click on the image!

Well, I can’t see a way to make a clipping mask from google, but I think I found a way to achieve the feature I wanted: