First you’ll want to upload an image somewhere, maybe with in wordpress, then grab the URL of the non resized image.

Go to your Apperance->Editor.
On the right side, look for “Stylesheet (style.css)” and click it.
Then look near the top for this:

body {
 font: 100% Arial, Helvetica;
 background: #FFF;
 color: #333;
 }

Add this

body {
 font: 100% Arial, Helvetica;
 background: #FFF;
 color: #333;
background-image: url('#');
}

where the “#” is the URL for the images for instance:

background-image: url('http://youraddress.com/images/backgroundimage.jpg');

Hit save and it should be good.