Hi Norbert, for this you would need to make use of the surfacePlacement methods on LayerSceneProperties. For example, the code below places a picture marker symbol at a height of 1000m above Edinburgh:
PictureMarkerSymbol* pms = new PictureMarkerSymbol(myImageUrl, this);
Graphic* g = new Graphic(Point(-3.1883, 55.9533, 1000., SpatialReference::wgs84()), pms, this);
overlay->graphics()->append(g);
LayerSceneProperties sceneProps = overlay->sceneProperties();
sceneProps.setSurfacePlacement(SurfacePlacement::Relative);
overlay->setSceneProperties(sceneProps);
Note that the sceneProps are returned by value not by reference so you need to set them back onto the overlay.
Hope that helps,
Luke