Featured Post Image is not shown in Post itself

Hey!

As far as I’ve seen, the Post Images are not shown in the post itself, which makes the post itself look very empty and boring:

Link

I thought about the option of creating a slider for each post.. but that is not very efficient.. As in other themes it is normal that the post image is shown in the post, too.
Is there any code I can put in my post template so that it will show the uploaded post image in the post?

And yes, the images are correctly shown as thumbnails, which you can see here:

Link

Which seems somehow really weird to me, as the images are ONLY shown as thumbnails and nowhere else in the theme!?