How exactly is featuredImage chosen? Can it be explicitly specified per post?

I am using numerous widgets to display recent posts, featured posts, tagged 
posts, etc. while displaying a featuredImage that is hosted externally on 
Flickr.

Currently, the widgets use post.featuredImage which is often the first 
image found in the post. Sometimes it uses the Youtube image thumbnail if 
there is any embedded Youtube video in the post.

For og:image meta tag, I found that it uses the first image in the post.

How does Blogger choose the image to be the featuredImage? Is it possible 
to explicitly define the featuredImage or thumbnail in each post, since the 
actual featuredImage is usually much bigger in size for a thumbnail?

Example URL, the Popular Posts widget on the right is currently using 
featuredImage as thumbnails.
*http://sgmacro.blogspot.com/2016/07/spider-photo-exhibition-senckenberg-natural-history-museum.html*

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