Continuing what Maker said, if there is indeed a portrait file included in your custom model, you have to import it correctly, if you do that, the portrait would probably show up. Try reading tutorials on this when you check models. There is a tutorial there about importing models, as well as portraits. If the model really doesn't have a portrait, then it would be just as Maker said, you can't do anything about it, unless someone makes a portrait you could use for that.