Xaringan: center image within two columns layout

I am using a two-column layout and I was wondering if I could center the image within the second column.

This is the YAML:

---
title: "Title"
subtitle: 
author: 
date: " </br> `r Sys.Date()`"
output:
  xaringan::moon_reader:
    lib_dir: libs
    css: ["default", "custom.css", "custom-fonts.css"]
    nature:
      beforeInit: "http://www.jvcasillas.com/ru_xaringan/js/ru_xaringan.js"
      highlightStyle: github
      highlightLines: true
      countIncrementalSlides: false
      ratio: "16:9"
---

The slide:

---
# Introduction

.pull-left[
- One bullet.
-  Another bullet.
- another 
- another
- another

]
.pull-right[

<img src="img/book.jpg" alt="drawing" width="200"/>

]

And this is how it looks like:

As you can see, the image is "hanging" from the top left corner of the second column. I was wondering if I could have it centered within the second column.

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