Why isn't my +Snippet image appearing?

Images that are too small or not square enough are not included in the +Snippet, even if the images are explicitly referenced by schema.org microdata or Open Graph markup.

Specifically, the 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.

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Link: https://developers.google.com/+/plugins/snippet/