It doesn't work either assigning an image to the back wall or panel:

      var backImage = new Image();
      backImage.Source = ImageSource.FromUri(new Uri("http://www.steema.com/uploads/news/xamarinforms_280x210.png"));

      tChart1.Walls.Back.Gradient.Visible = false;
      tChart1.Walls.Back.Image = backImage;

      tChart1.Panel.Gradient.Visible = false;
      tChart1.Panel.Image = backImage;

nor drawing it directly on the canvas:

    void tChart1_BeforeDrawSeries(object sender, Graphics3D g)
    {
      Rectangle chartRect = tChart1.Chart.ChartRect;
      var backImage = new Image();
      backImage.Source = ImageSource.FromUri(new Uri("http://www.steema.com/uploads/news/xamarinforms_280x210.png"));

      g.Draw(chartRect, chartRect, backImage, false);
    }