If you wish to display an image (be it the featured image or any other) on specific single posts, you can include the image directly at the beginning of the posts themselves.
You might be able to find a plugin which provides the functionality of displaying the featured image via a shortcode, but it sounds to me a more complex solution than simply inserting the images in the content.
Repository themes are not allowed by the guidelines to include shortcode functionality.
There is no Septera Plus theme yet.