Mixing explicit and implicit joins fails with "There is an entry for table ... but it cannot be referenced from this part of the query"

SELECT
      i.*, 
      r.name AS roomname, 
      c.name AS cat, 
      p.key AS imgkey, 
      p.extension AS imgext
   FROM 
      items i, 
      rooms r, 
      categories c 
         LEFT JOIN photos p 
            ON p.referencekey = i.key 
   WHERE 
          i.room = r.key 
      AND r.key = 663308 
      AND i.sitekey = 32201 
      AND c.key = i.categorykey

The above query when executed returns following error.

ERROR: invalid reference to FROM-clause entry for table "i"

LINE 1: ...tegory c LEFT JOIN photos p ON p.referencekey = i.key WHER...

HINT: There is an entry for table "i", but it cannot be referenced from this part of the query.