Your sample is the code itself. Template file location and what to edit in that template file are provided above. There is only 1 usage of $insert in that template file and it's responsible for outputting the article. You'll notice the ->get uses against the article object in the $insert usage. Specifically the following.
Article Title: $article->get( 'title', null, GetterInterface::STRING )
Article Intro: $article->get( 'introtext', null, GetterInterface::HTML )
There is no specific usage for pulling out a single image for you to format it. It's not that simple. You'd need to parse for the image in the introtext, pull it out, then place it in the HTML differently or just hardcode whatever implementation you're needing as you can output whatever you want with $insert.
Your alternative is to just format your article how you want it to display in CB Activity and it'll output like that since it just directly outputs introtext.