Choose what image to use for “Image” and “Attachment” post types if post has several images
Choosing what image to use for “Image” and “Attachment” post types
You can select what image to use for autoposting if your post has several images.
Short video demo of auto-posting to Google+ with selected images. Video shows that image selection is working for both “Image Posts” and for thumbnails for “Text Posts with “attached” blogpost”