Randomizing two sets of images, dependent on mobile or desktop

Site URL: http://jlinnertarchitecture.com

Hi all, 

I am trying to make it so that when a visitor sees my homepage, they get a random image every time.

With that being said, I would like for the image, when viewed on mobile, to be chosen from a set of vertically-oriented shots, and when viewed on desktop, to be among a set of landscape-oriented shots. 

I have seen some people mentioning that they have a different home page for mobile, or an index of it or something. I'm fairly competent with HTML generally, but new to CSS and Squarespace. So I don't totally understand the structures and backend. 

Can anyone help offer guidance with how I can achieve this? Ideally, I would link the images from ones hosted on other pages in my squarespace website.

Thank you very much. This is a screenshot of how my site is organized now.