CREATE TABLE test1
AS
SELECT 1 idx, 1 grp, '김태희' name, NULL image
UNION ALL SELECT  2, 1, '송혜교', NULL
UNION ALL SELECT  3, 1, '전지현', 'jihyun.jpg'
UNION ALL SELECT  4, 2, '한예슬', NULL
UNION ALL SELECT  5, 2, '아이유', 'iu.jpg'
UNION ALL SELECT  6, 2, '이민정', 'minjung.jpg'
UNION ALL SELECT  7, 2, '수지'  , 'suzi.jpg'
UNION ALL SELECT  8, 3, '태연'  , NULL
UNION ALL SELECT  9, 3, '윤아'  , 'yoona.jpg'
UNION ALL SELECT 10, 3, '유인나', NULL
UNION ALL SELECT 11, 3, '박보영', NULL
UNION ALL SELECT 12, 4, '신세경', 'kain.jpg'
UNION ALL SELECT 13, 4, '한가인', NULL
UNION ALL SELECT 14, 4, '김사랑', 'sarang.jpg'
;

SELECT a.idx, a.grp, a.name, a.image
  FROM (SELECT a.idx, a.grp, a.name, a.image
             , @rn := CASE WHEN @grp = a.grp THEN @rn + 1 ELSE 1 END rn
             , @grp := a.grp
          FROM (SELECT a.idx, a.grp, a.name, a.image
                  FROM test1 a
                  LEFT OUTER JOIN
                       (SELECT grp, MAX(idx) idx
                          FROM test1
                         WHERE image IS NOT NULL
                         GROUP BY grp
                        ) b
                    ON a.idx = b.idx
                 ORDER BY a.grp
                     , CASE WHEN b.idx IS NOT NULL THEN 1 ELSE 2 END
                     , a.idx DESC
                ) a
        ) a
 WHERE rn <= 3
;