changing backgound

[jisafriend]

I am sure it's a little thing I am missing. But I have tried to put an image behind the piece of the template which has the article tag, that is white, to no avail.
Please tell me where to put the background css tag, in the body css, the css for the main class? I've tried it everywhere.
Here is how I am using Initialzr.
http://nicodonationpage.site44.com/