Chartjs-Is it possible to add image behind doughnut chart with transparency color in ng2chart?

Background in Highcharts is an SVG element, so if you want to add image background to it you should apply defs pattern:

events: {
  render: function() {
    var chart = this,
      renderer = chart.renderer,
      bg = chart.plotBackground;

    var pattern = renderer.createElement('pattern').add(renderer.defs).attr({
      width: 1,
      height: 1,
      id: 'bg-pattern',
      patternContentUnits: 'objectBoundingBox'
    });

    renderer.image('https://avatars3.githubusercontent.com/u/15981345?s=400&v=4', 0, 0, 1, 1).attr({}).add(pattern);

    bg.css({
      color: 'url(#bg-pattern)'
    });
  }
}

Points opacity can be set via CSS:

.highcharts-point {
  opacity: 0.5;
}

Live demo: http://jsfiddle.net/kkulig/694gs6qv/

Docs reference:
https://www.highcharts.com/docs/chart-design-and-style/gradients-shadows-and-patterns

API reference:

• https://api.highcharts.com/highcharts/chart.events.render
• https://api.highcharts.com/class-reference/Highcharts.SVGRenderer