How to save an image of the chart on the server with highcharts?

It could be done really easy with PhantomJS. You can render Highchart chart and save it to SVG, PNG, JPEG or PDF. The example below renders a demo Highcharts diagram to SVG and PDF at the same time:

var system = require('system');
var page = require('webpage').create();
var fs = require('fs');

// load JS libraries
page.injectJs("js/jquery.min.js");
page.injectJs("js/highcharts/highcharts.js");
page.injectJs("js/highcharts/exporting.js");

// chart demo
var args = {
    width: 600,
    height: 500
};

var svg = page.evaluate(function(opt){
    $('body').prepend('<div id="container"></div>');

    var chart = new Highcharts.Chart({
        chart: {
            renderTo: 'container',
            width: opt.width,
            height: opt.height
        },
        exporting: {
            enabled: false
        },
        title: {
            text: 'Combination chart'
        },
        xAxis: {
            categories: ['Apples', 'Oranges', 'Pears', 'Bananas', 'Plums']
        },
        yAxis: {
            title: {
                text: 'Y-values'
            }
        },
        labels: {
            items: [{
                html: 'Total fruit consumption',
                style: {
                    left: '40px',
                    top: '8px',
                    color: 'black'
                }
            }]
        },
        plotOptions: {
            line: {
                dataLabels: {
                    enabled: true
                },
                enableMouseTracking: false
            },
            series: {
                enableMouseTracking: false, 
                shadow: false, 
                animation: false
            }
        },
        series: [{
            type: 'column',
            name: 'Andrii',
            data: [3, 2, 1, 3, 4]
        }, {
            type: 'column',
            name: 'Fabian',
            data: [2, 3, 5, 7, 6]
        }, {
            type: 'column',
            name: 'Joan',
            data: [4, 3, 3, 9, 0]
        }, {
            type: 'spline',
            name: 'Average',
            data: [3, 2.67, 3, 6.33, 3.33],
            marker: {
                lineWidth: 2,
                lineColor: 'white'
            }
        }, {
            type: 'pie',
            name: 'Total consumption',
            data: [{
                name: 'Andrii',
                y: 13,
                color: '#4572A7'
            }, {
                name: 'Fabian',
                y: 23,
                color: '#AA4643'
            }, {
                name: 'Joan',
                y: 19,
                color: '#89A54E'
            }],
            center: [100, 80],
            size: 100,
            showInLegend: false,
            dataLabels: {
                enabled: false
            }
        }]
    });

    return chart.getSVG();
},  args);

// Saving SVG to a file
fs.write("demo.svg", svg);
// Saving diagram as PDF
page.render('demo.pdf');

phantom.exit();

If you save the code as demo.js, then just run bin/phantomjs demo.js to generate demo.svg and demo.pdf