How to add one image above the Navigation Bar in the yml file?

I am editing one webpage using Rmarkdown under Rstudio. I want to add one image at the top of the webpage (above the navigation bar). But I didn't figure out how to do it.

What I did is to edit the file ".yml". The file ".yml" looks like this

name: "TEST"
title: "TEST"
description: "TEST"
output_dir: "."
navbar: 
   right:
        - text: "HOME"
          href: index.html
        - text: "TEST"
          href: test.html

output:   html_document:
        theme: readable
        highlight: textmate
        include:
          in_header: header.html
          after_body: footer.html

Any suggestion on how to add one image above the navigation bar? Just like the below example (http://jsfiddle.net/7D389/). Thanks so much.