Understanding linking tables in a many-to-many relationship
Actually you don't need a special value for people that aren't there
When you select a photo andand you don't know if there aren't any people in it, you make
SELECT p.*,pe.*
FROM Photo p 
   LEFT JOIN reftable r ON p.id = r.p_id 
   LEFT JOIN peope pe ON  r.pe_id = pe.id 
WHERE p.id = 123

So even when there is no entry in the Relationship table, you get an photo.
If you have one person and want to find all photos that they are in
SELECT pe.*,p.*
FROM peope pe  
   INNER JOIN reftable r ON r.pe_id = pe.id
   INNER JOIN Photo p  ON p.id = r.p_id  
WHERE pe.id = 123

You need here a relationship table, because many persons could be on a photo or one person can be in many photos