Is there an official Otaku day?
I've been searching for official answers, as you said, this is probably new, started on 2012 (or at most, 2011). All what I could find are some personal posts.

I have searched Google for similar images of the one below (the one below is used to advertise for OtakuDay):



And so, I found the original drawing here with title "coffee-kizoku-shibuya_rin-the_idolm-ster-the_idolm", which is a fan-art of an anime/manga character, but it has nothing to do with the OtakuDay.

I've searched twitter for #OtakuDay, there is really nothing official even on that hash tag, all I found is personal posts again (and it is probably started on 2011).

So far, the only official (non-personal) thing is actually what you posted from the AnimeNewsNetwork.com:

2nd Annual International Otaku Day = 3rd Saturday of August = "August 18, 2012"

3rd Annual International Otaku Day = 3rd Saturday of August = "August 17, 2013"

So, the 1st Annual International Otaku Day = 3rd Saturday of August = "August 20, 2011"

As for the Dec 15 it's made up in 2010 and it has been spreading since then. I finally could find the source, the oldest and very first one is this: here in tumblr posted on 14th Jul 2010

EDIT:

So, the 1st Annual International Otaku Day - 3rd Saturday of August = "August 20, 2011" is by the ANN. And the December 15th is World Day of Otaku is made by anime/manga fans.