First, create a column with two image modules (one for each image) and then a show / hide button below them, like so:
Use one image module for each of the two images and set each to open in a light box. Give each a CSS ID, e.g. "image1" and "image2"
Configure the show / hide button to hide image2 initially:
And to toggle the two image modules on click:
You might also want to set the transition duration to "0" so that there is no animation during the change. You can do that with this option: