public static IDisposable AddOverlay( 
   MapView mapView,
   CIMGraphic graphic,
   double referenceScale
)

//Defined elsewhere
private IDisposable _graphic = null;
public async void GraphicOverlaySnippetTest()
{
  // get the current mapview and point
  var mapView = MapView.Active;
  if (mapView == null)
    return;
  var myextent = mapView.Extent;
  var point = myextent.Center;

  // add point graphic to the overlay at the center of the mapView
  _graphic = await QueuedTask.Run(() =>
  {
    //add these to the overlay
    return mapView.AddOverlay(point,
        SymbolFactory.Instance.ConstructPointSymbol(
                ColorFactory.Instance.RedRGB, 30.0, SimpleMarkerStyle.Star).MakeSymbolReference());
  });

  // update the overlay with new point graphic symbol
  MessageBox.Show("Now to update the overlay...");
  await QueuedTask.Run(() =>
  {
    mapView.UpdateOverlay(_graphic, point, SymbolFactory.Instance.ConstructPointSymbol(
                                  ColorFactory.Instance.BlueRGB, 20.0, SimpleMarkerStyle.Circle).MakeSymbolReference());
  });

  // clear the overlay display by disposing of the graphic
  MessageBox.Show("Now to clear the overlay...");
  _graphic.Dispose();

}

// get the current mapview
var mapView = MapView.Active;
if (mapView == null)
  return;

//Valid formats for PictureURL are:
// e.g. local file URL:
// file:///<path>
// file:///c:/images/symbol.png
//
// e.g. network file URL:
// file://<host>/<path>
// file://server/share/symbol.png
//
// e.g. data URL:
// data:<mediatype>;base64,<data>
// data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEASABIAAD/2wBDAAU ...
//
// image/bmp
// image/gif
// image/jpeg
// image/png
// image/tiff
// image/x-esri-bglf

var pictureGraphic = new CIMPictureGraphic
{
  PictureURL = @"file:///C:/Images/MyImage.png",
  Box = envelope
};

IDisposable _graphic = mapView.AddOverlay(pictureGraphic);

internal class AddOverlayWithText : MapTool
{
  private IDisposable _graphic = null;
  private CIMLineSymbol _lineSymbol = null;
  public AddOverlayWithText()
  {
    IsSketchTool = true;
    SketchType = SketchGeometryType.Line;
    SketchOutputMode = SketchOutputMode.Map;
  }

  protected override async Task<bool> OnSketchCompleteAsync(Geometry geometry)
  {
    //Add an overlay graphic to the map view
    _graphic = await this.AddOverlayAsync(geometry, _lineSymbol.MakeSymbolReference());

    //define the text symbol
    var textSymbol = new CIMTextSymbol();
    //define the text graphic
    var textGraphic = new CIMTextGraphic();

    await QueuedTask.Run(() =>
    {
      //Create a simple text symbol
      textSymbol = SymbolFactory.Instance.ConstructTextSymbol(ColorFactory.Instance.BlackRGB, 8.5, "Corbel", "Regular");
      //Sets the geometry of the text graphic
      textGraphic.Shape = geometry;
      //Sets the text string to use in the text graphic
      textGraphic.Text = "This is my line";
      //Sets symbol to use to draw the text graphic
      textGraphic.Symbol = textSymbol.MakeSymbolReference();
      //Draw the overlay text graphic
      _graphic = this.ActiveMapView.AddOverlay(textGraphic);
    });

    return true;
  }
}