Line 13: An empty MC is created and named boxes which, as the name suggests, will holds all the box masks.
Line 14: Another empty MC is created which will hold the top layer of images
Line 15: The boxes MC is then set to mask the top MC
Line 16: Another instance of the box MC is created, and named as mask. Since we're going to be lining up all the images horizontally, they need to be masked so that only 1 is showing at a time. So this mask will be used to mask the bottom layer of images.
Line 17 - 18: The mask is set to have the same width and height as each image.
Line 19 - 20: The X & Y position of the mask is set so that it is covering the space where we want the image to appear.
Line 21: Another empty MC is created to hold the bottom layer of images
Line 22: The large mask MC is set to mask the bottom MC
Line 22: A boolean variable will stored whether or not to shrink the box MCs