I actually make a small comment about this a long time ago in the suggestions for improvement thread. I'll just copy and paste what I said.
Official just means something was published in a commercial capacity.
And canon is a term for what's factual in a fictional setting. Promotional art doesn't usually fall under that. There are some weird exceptions like promotional art showing up in Nanoha and Fate's room during Strikers.
Basically, something being official does not automatically means it's canon.
last edited at Jul 4, 2015 1:26PM