public string PictureURL {get; set;}

// 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);