title: Highcharts background image size option
body: <p>I'm using Highcharts to render charts on webpage. Now I found an issue if I use <code>min-width</code> and <code>max-width</code> CSS option on container div:</p>
  
  <pre><code>&lt;div id="container" style="min-width: 310px; max-width: 960px; height:400px;"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>I added a background image with this method:</p>
  
  <pre><code>Highcharts.theme = {
      chart: {
          backgroundColor: '#FFFFFF',
          plotBackgroundImage: '/path/to/mybackgroundimage.png'
      }
  };
  </code></pre>
  
  <p>This image has 960px width and 400px height and it shows well on desktops, but if I check it on mobile devices the image's apect ratio has gone and the image become distorted.</p>
  
  <p>Is there a <code>backgroundSize:'cover'</code> or something like this to fix the image position and size on different screen widths? </p>