Embed pictures from posts and links

I think you are asking about the Twitter Cards feature of the Twitter plugin for WordPress and how links to article pages appear in Twitter’s link previews.
https://dev.twitter.com/web/wordpress/cards

An image included in the post is output as a twitter:image meta element value for indexing by Twitter’s cards feature. The image highlighted in this Twitter Cards markup element may be the post’s featured image, an image attached to the post, or an image in a gallery included in the post content.

Is there a type of image you are including in an article that does not appear as a twitter:image meta element in your article HTML?