Hi
I used the code below -- you need to get an add-on for chrome (Squarespace id finder) to allow you to find the section id to be put in where the bold text is in the css below.
On the page you will need to add 2 images in 2 separate sections
1 - for the desktop image
2 - for the mobile image
once you edit the section id in the code below then copy and paste the code below in DESIGN / CUSTOM CSS (don't for get to save once added)
IF YOU GET STUCK FIRE ME OVER A LINK TO THE PAGE AND ILL WRITE YA CODE FOR YA
//VARIABLES//
@mobile: ~"only screen and (max-width: 640px)";
@desktop: ~"only screen and (min-width: 760px)";
//hide mobile section on desktop//
@media @mobile {section[YOUR DATA GOES HERE"]{display:none!important;}}
//hide desktop section on mobile//
@media @desktop {section[YOUR DATA GOES HERE"]{display:none!important;}}