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?

Concatenation

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

Ninetta

Thank you! This is correct, as I just tested 119px vs 120px and the latter worked. I read through the FAQ but I somehow missed this question, perhaps because there is no space between the paragraphs. – Concatenation