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: