Add an image background to the promotional pop up

Hello Squarespace,

I would like to add an image background to the promotional pop up. Please could someone share some code? I've seen lots of people talking about further tweaks to images but can't find out how to add an image in the first place. 

And would anyone know the aspect ratio the image background should be?

https://www.weemovers.co.uk

Thanks 🙂