I am changing the theme Black Bike - and on the homepage, I want to update an image which works as expected on the mobile size but reverts to the original image at the larger screen size.

Thoughts?

I did read about starting new might be faster than updating a theme - but this starting point offers a layout function (sliding content areas) that works for the site I'm trying to re-do.

Found this from Eric R. which works!

Click on Actions upper left, then Toggle Break Points, then select Default Mode.
You'll then be able to change the image at the higher break points.