Not in and Exists not working as i expect

all. My question is easy, but I can't solve it by myselft. I have 2 tables: et_pics.ob_no where ob_no is user id and et_thanks_2014 where thnk.e_to is a link to et_pics.ob_no. I need to find out ob_no in et_pics who absent in e_to in et_thanks_2014.

SELECT pics.ob_no, thnk.e_to FROM et_pics pics 
left join et_thanks_2014 thnk on thnk.e_to = pics.ob_no
WHERE e_to is null

This code works, but I thinks it's not the best way to solve my task. I've tried to solve it with IN predict:

SELECT pics.ob_no FROM et_pics pics 
WHERE pics.ob_no  in ((SELECT e_to FROM et_thanks_2014))

and not exists

SELECT ob_no from et_pics
WHERE not exists (SELECT DISTINCT (e_to) FROM et_thanks_2014 thnk)

but both returns nothing. Why?