In order for that first image to be set in the Twitter Card for the article, the twitter:image tag needs to be set correctly on that page. The easiest way for you to do this is 1) enable the 'Open Graph - Content' plugin. JFBConnect will then automatically pick the first image of the article for the Open Graph / Twitter tags. If you want further control of where to pull the image or description from, I would suggest you create an object in the JFBConnect backend > Social Meta > Objects area for your articles.
Also, please make sure in JFBConnect backend > Social Meta > Configuration that you have the Twitter Cards setting enabled.
Once the tags are shared correctly, then anything shared to Twitter should show the correct image/description preview. However, if this is unclear or you're having problems setting it up after following the above steps, let us know.
-Melissa