How to make margin percentages the same on mobile

Here’s my site: https://pennettahouse.com/

I’d be very grateful if anyone could give me any ideas as to why the image (at the top of a wp page within the site) appears to have a larger margin at the top than it does in a mobile browser?

I’ve been using:

body {
margin: 15% 0;
padding: 0;
}

in an attempt to get the two looking roughly centralised. I’m not quite sure what this is taking a percentage of, I’m just intuitively trying things out.

(lol)

Thanks a lot!

(The theme’s BlankSlate)