I am looking for a sample of Javascript code to visualise pictures in a tooltip of a pie chart from Highcharts.

I would like to see different pictures according to the sectors I am navigating on...

Thanks in advance.

According to the docs, the tooltip.formatter is:

... a subset of HTML. Unless useHTML is true, the HTML of the tooltip is parsed and converted to SVG, therefore this isn't a complete HTML renderer. The following tags are supported: <b>, <strong>, <i>, <em>, <br/>, <span>.
Spans can be styled with a style attribute, but only text-related CSS that is shared with SVG is handled.

So you should set tooltip like this:

<!-- language: lang-js -->
tooltip: {
  useHTML: true,
  formatter: function () {
    return '<b>' + this.point.name + "</b>: " + this.point.percentage + "%<br/>" + 
           "<img src='" + this.point.img + "'></img>";
  }
},

Here's a demo in Stack Snippets

<!-- begin snippet: js hide: true console: true babel: false --> <!-- language: lang-js -->
Highcharts.chart('container', {
    chart: {
        plotBackgroundColor: null,
        plotBorderWidth: null,
        plotShadow: false,
        type: 'pie'
    },
    title: {
        text: 'Favorite Fruits'
    },
    tooltip: {
      useHTML: true,
      formatter: function () {
        return '<b>' + this.point.name + "</b>: " + this.point.percentage + "%<br/>" + 
               "<img src='" + this.point.img + "'></img>";
      }
    },
    plotOptions: {
        pie: {
            allowPointSelect: true,
            cursor: 'pointer',
            dataLabels: {
                enabled: false
            },
            showInLegend: true
        }
    },
    series: [{
        name: 'Fruits',
        colorByPoint: true,
        data: [{
            name: 'Apple',
            y: 50,
            img: "https://i.imgur.com/MmK9Xkc.png"
        }, {
            name: 'Banana',
            y: 25,
            img: "https://i.imgur.com/0G6GXWf.png"
        }, {
            name: 'Orange',
            y: 15,
            img: "https://i.imgur.com/Dv4KoD5.png"
        }, {
            name: 'Watermelon',
            y: 10,
            img: "https://i.imgur.com/2LN8PfD.png"
            
        }, ]
    }]
});
<!-- language: lang-html -->
<link href="https://cdnjs.cloudflare.com/ajax/libs/highcharts/6.1.2/css/highcharts.css" rel="stylesheet"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/highcharts/6.1.2/highcharts.js"></script>


<div id="container" style="min-width: 310px; height: 400px; max-width: 600px; margin: 0 auto"></div>
<!-- end snippet -->