Question about Imgage in Demo Page

Hi there,

It's just a featured image set to above content area:
https://docs.generatepress.com/article/adjusting-the-featured-images/

Let me know if this helps.

Okay, then this means, that all or no pages have a featured image, right?
One can not put such an image only for one special page?

Does this also works on Blog Homepage?

Not sure if I understand. Featured image is per page basis so you can have a different featured image for different pages.
And not all pages need to have a featured image as they are uploaded separately:
https://docs.generatepress.com/article/adding-featured-images/

For the main blog page I would recommend using the page header add-on and assign in global location:
https://docs.generatepress.com/article/page-header-global-locations/

Let me know if this helps.