Header image is cropped

Using Bento and the extended page header features, I want to display a header image using my home page, rather than using the option of theme customizer (home page settings).
Unfortunately, the chosen image (1200 x 800) is displayed in a “post-header” which size is 1426 x 380.467. As a result I can only see less than the top half of the image.
This behavior is exactly the same as if I had declared my header image through the home page settings in the theme customizer.

But what I need is to get the whole image displayed as a header image, exactly the same as Bento home page (black and red). I noticed that, on Bento home page, the post-header size is 1426 x 636,6 (on my computer): same width but height is almost twice as on my home home page.

Is it possible to adjust the post-header size to the proportions of the header image and how ?

You can figure out the situation at the url
“https://test-wp.minerve-occitanie.fr/”
(empty slider in the home page is normal as the images were deleted).

Thanks for your answer,
Best regards,

Jean Cordier