title: How to add an image on ggplot background (not the panel)?
body: <p>How do I use an image as the background for a ggplot2 plot?</p>
  
  <p>For example, the code:</p>
  
  <pre><code>mtcars %&gt;% 
    ggplot(aes(cyl)) + 
    geom_bar() + 
    theme(plot.background = element_rect(fill = "black"))
  </code></pre>
  
  <p>It results in the following:
  <img src="https://i.stack.imgur.com/TWn0r.png" alt="plot"><br>
  and the background color is black.</p>
  
  <p>How can I choose an image to be the background, and not only choose its color?</p>