Add Background Image

Hi

My site is still a work in progress but currently the theme only has the option of setting a background color not a background image, can anyone let me know the code and which file to edit in order to be able to replace the current white background with an image.

Site is https://www.lmlakelanddesign-clients.co.uk/

Also how can I increase the width of the top block where the logo sits? Th logo I will be adding is too big to fit in the current space.

Thank in advance

D