To proof my words i just did as you've suggest:

1. two non-administrative accounts was created - A and B, both on different Accounts, both with completed profiles, both have no one friend on site.

2. user A created  Closed Group.

3. user B can see in Common Site Timeline that Closed Group was created by user A, which is very OK so far

4. user A posts couple of words and pic in the Closed Group,  under Public privacy choice

5. user B can see the post in Common Site Timeline, can read the words and can see the pic, even in enlarge. Can not : to enter the post itself in group.

BUT still CAN see the pic and the text despite being non-member and non-administrator. 

As I said with my pleasure two weeks ago.