The New Map Viewer Beta does not allow adding Map Image Layers to the map view. Without the capability of having this option to add to the map it will not be possible to map image layers. Please make this possible for Portal users as well, because this capability is only available for ArcGIS Online users.
You have to add a Map Image Layer in the Classic map viewer. You can then open the web map in the Beta Map Viewer, and the map image layers will still be there.
Yes. AGOL usually shows some of the forthcoming features and improvements that you can expect to see in Portal, and the process works there. I expect this will be in 10.9.1, but we'll see.