WordPress, bootScore, and/or my browser (chrome) act cleverly:
If I call the list of posts the first time (meaning technically: calling the index.php offered by bootScore), the medium pictures are used and fitted (upscaled) on the browser in the space bootScore has planned to offer.
But after having called a post the first time (in which the originally uploaded picture is presented as Featured Image) and having come back to the post list (reload), now the originally uploaded picture is also presented in the respective entry of the post list.
It’s taken from the browser cache. You can prove that by emptying your browser cache and repeat these steps.