Why does Google ignore my +1 page thumbnail?

Question

Google +1 button generator implies I need to specify <meta itemprop="image"> for the thumbnail.
Documentation page says the following order is respected:

1. Schema.org microdata

   <img itemprop="image" src="..."></img>
   

2. Open Graph protocol

   <meta property="og:image" content="..."/>
   

Neither of three, even combined, worked for me.

Google Plus sharer ignores the image of my choice and shows a different image instead.
This is not a caching issue, as I have changed subdomains a couple of times just to check.

It looks like Google +Snippet prefers larger square images.
I'm building an extensible engine so I need to know the requirements for sure.

What is the minimum size for a Google+ thumbnail?
Does it have to be square, or how close to square does it have to be?

Answer 1

According to the FAQ:

[T]he height must be at least 120px, and if the width is less than 100px, then the aspect ratio must be no greater than 3.0