Jon,
If you’ll go to the appearance section and select background, you can add a custom image there.
Another way to do it, that would be to upload the image you’d like to use by going to Media and selecting “Add New.” After the file finishes uploading, copy the URL to that file that WordPress gives you.
Next go to the appearance section and select edit. From the list of files on your right, choose style-blue.css. Look for this spot:
body { margin: 0 auto; background: #142634 url(images/bg-blue.jpg) repeat center top; }
and replace the images/bg-blue.jpg with the URL you copied earlier. Now your new image will be visible and you won’t be loading any extra background images either (speedier site).
If you have any trouble, let me know.