New Post: Background image in full screen

If you are looking for some kind of option in the C1 Console - to set the background image for all pages or a single page, I am afraid there is none.

You can set the backgound image via CSS or LESS. Something like that:

body {
      background: url('background.png');
}

However, this is general CSS stuff not specific to Composite C1. Please refer to CSS manuals on the web.

You can create or edit CSS / LESS files from the Layout perspective:

~/Frontend/Styles/

From this perspective, you can also edit the page templates and set the background image there
( if you prefer to set attributes on the <body/> elements.)

But again, this is not specific to Composite C1.