want to change the background to a full screen image

Hi folks

I have been looking through the may different ways described on how to change the background image in bp-social theme, now I can't see the woods for the trees.

I have activated the child theme and tried a few css classes, nothing seemed to work,

so, I read, do not edit anything but child theme, ok, clear, then i read this:
https://premium.wpmudev.org/forums/topic/how-do-i-change-the-background-of-the-site

Can someone give me a concise very clear explanation on what where and how I can make the background of bd-social a full screen image please?

Thanks guys.

JT