I am making a custom unit from scratch and I have gotten to the part where I want to set its portrait. For simplicity, I tried using the same model for the portrait as for the unit itself. However, the model appeared way too large, making the entire portrait a solid color. So, I made another model (with the same .m3 file) but with the scale factor 10x smaller. Unfortunately this had no effect whatsoever.
My suspicion is that you need a special type of model for portraits, maybe with a camera placed somewhere.