Use featured image as header in Twenty fourteen

Hi!
I think this is suppose to be the default setting (the featured image being set as the page’s header) but I must have messed things up on the way. Can anybody help me with this problem? I have googled and searched the forums to no avail – using

<?php elseif ( is_page ( xx ) ) : ?>
<img src="https://path/to/your/image.png" width="<?php echo get_custom_header()->width; ?>" height="<?php echo get_custom_header()->height; ?>" alt="home"><?php endif; ?>

only resulted in the image showing up above the header, not replacing it :/

(I have created a child theme so no need to point that out ?? )

https://awkwardsilen.se/raa/rallylydnad-med-siv/ This is an example page of the featured image now being shown below the header instead. And please, explain it as if I was a five year old child. I’m not an especially good coder. ;D