Help with background and keeping it centered

Hello,

I would like to set a background behind my content and have it remain in the exact same position/size behind the content at all times, regardless of the size of the browser. Ideally, I would like the black background image to remain like this https://i.imgur.com/k9V8Qky.png and keep the auto margin as well (the background image will eventually be white, I simply put that one up for visuals).

However, when I resize the browser beyond the content – the background image moves along with it and readjusts like this https://i.imgur.com/fDlvTLY.png

I’ve been searching for a while, but I cannot find anything that helps.

Thanks!