title: Highcharts warning: Invalid attribute 'src' in config
body: <p>Hi I started getting the warning in Title. It used to work but now it does not work anymore :(
  Any tips would be greatly appreciated!
  Here is a jsfiddle. When hovering mouse over one of the two dots, the error is triggered from line 204 in the JS. It should display an image: <a href="https://jsfiddle.net/yngtp9xv/" rel="nofollow noreferrer">https://jsfiddle.net/yngtp9xv/</a></p>
  <pre><code>tooltip: {
    useHTML: true,
    borderWidth: 0,
    shadow: false,
    formatter: function() {
    return '&lt;img src=&quot;./Earthmap.jpg&quot; width=&quot;180&quot; height=&quot;90&quot; /&gt;'
    },
  </code></pre>