UPDATE tbl_users 
SET img = CASE gender 
        WHEN gender=1 THEN 'client_files/44073762e25b9fed53b/userPic/default/male.jpg' 
        WHEN gender=2 THEN 'client_files/44073762e25b9fed53b/userPic/default/female.jpg' 
        ELSE 'client_files/44073762e25b9fed53b/userPic/default/male.jpg' 
      END