I am not sure how that would fit in to the theme concept which is entirely set toward large, prominent images that span the entire page. Of course, anything can be done, but you'd need a much clearer definition of how you imagine this to look - a mockup or wireframe for instance. Second, and more importantly, the change would be quite considerable and out of scope for a public forum answer - I suggest you contact a Shopify Expert if you really wish to go forward with this customisation.
Well I see you're hell bent on having it and having it for free 😀
There's a reason why the app you mention is paid - it took considerable effort to build it. There's also a reason why I cannot offer an answer on how to do this for this particular theme - it would take considerable effort and that's not within my scope of free community service. Of course, mileage may vary for others.
I can make one suggestion though if you're up for the journey. There are several free themes from Shopify that have the exact same, albeit a little less configurable through UI, image sliders for product detail pages. Simply pick one, install the theme and start studying the code then copy paste, break stuff, pull out hair, fix stuff, pull out more hair and eventually you'll get there. The main bit to look out for is usually the theme.js file and within that the functions productImageSwitch and switchImage though that varies a little from theme to theme. Oh and the CSS varies a lot.
Hope this helps a bit, good luck!