You can drag a picture up from un-named people to a stack in named people. The pic usually stays on top of the stack until you add more images.
I couldn't get this technique to work. I went to the Named Person and choose the photo that I liked and removed the name. This did put the picture into the Unnamed People section with a "?" in the name. I did drag the pic to correct stack in the Named People section...This did add the picture, but did not change the image in the Named People section.