Google is using the wrong post image

I found that when sharing a link on google+, the featured image displayed is wrong; in fact in this case it’s showed the image used as main image of the theme.

I solved this issue setting the following metatag:

<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<meta itemprop="image" content="<?php echo $image[0]; ?>">
<?php endif; ?>

Hope this could help someone.