I had the same problem and found a solution (PSE 10):
1) Search a photo of the person with a keyword tag.
2) Click on the tag in the photo to start the face recognition (I hope this is the right translation).
3) Select one of the photos to be used for the tag and move it by drap & drop over the tag photo to be replaced on the left upper side.
Great! This is the solution I was looking for.
Just to clarify the method:
- Select a photo with the tag you want to change the icon.
- Double click it in the organizer to open it on the one photo view
- Click the tag of the person that you want to change the icon
- The People Recognition - Confirmation dialog opens.
- Select the confirmed tab.
- Choose the photo that you want for the icon from the confirmed photos and drag over the current one, on the upper left corner of the dialog.