Adding a background image

I started a new subreddit and I'm trying to add a background image. I uploaded the image and saved it and it says

link: url(%%arrow%%)

I have then tried copy-pasting a few things from this subreddit but none seem to work. For example,

/*Background image*/
body {
  background: url(%%arrow%%) no-repeat fixed center center
}

gives

syntax error: "CSSValue: Unknown syntax or no value: u'{background:url(%%arrow%%)no-repeatfixedcentercenter}'."

I'd like the image to be centered. It's 500x500, if that makes any difference.

Can anyone help me out?

Thanks in advance!