Sizing responsive images for posts and front page in child-of-child theme

Hi

I’ve created a responsive website for my wedding, and am struggling with the images and how they are cropped / appear on computers vs mobile devices.

The site is jamwedding.uk, and is a child of the Cubic theme – itself a child of the Boardwalk theme.

The front page crops images on a PC/Mac, and distorts the images on an iPhone. The original images are 800w x 381h.

Rather than crop them, I would like to show the full image but with a white background making up the remaining space.

For the static post image (featured image) I would like to see the whole image (800w) on both OS and mobile devices (although resized as appropriate for iOS), aligned to the width of the text column below. Currently I can either get it to present as full-screen width on the PC and cropped on the iPhone, or vice versa.

Apologies I am new to all this, and have spent hours trawling through similar threads. Not sure whether the theme being the child of a separate theme is casuing a problem, or whether the style.css is unnecessarily overriding WP4.4’s new responsive features, or whether I’m just not editing the right thing… but I cannot find how to control this effectively.

Any assistance would be appreciated!

Thanks