Problem with header image size and resolution.

I have done this: https://www.screencast.com/t/2ZImkvYhTd
With a fairly good resolution image, as you can see here: https://raizdiario.es/wp-content/uploads/2018/06/WhatsApp-Image-2018-06-29-at-18.30.05.jpeg
But when I try to put this image as a header image, it degrades its resolution to absurd levels, as you can see here: https://raizdiario.es/
Plus the image is cropped and not displayed in full.

I’ve changed the type of header because I can’t have such a weird header, so if you need proof of what I was saying, please let me know and I’ll put it back.

I’ve changed the image to a new one with the correct size of the website, and it keeps beeing zoomed. You can check it here, this is the original one: https://raizdiario.es/wp-content/uploads/2018/07/WhatsApp-Image-2018-07-01-at-20.48.54.jpeg and this is the one that is on my landing page: https://raizdiario.es/wp-content/uploads/2018/07/WhatsApp-Image-2018-07-01-at-20.48.54-300×44.jpeg

Okey jajaja, I managed to change this by changing the url of the picture, now it has de correct size, but any way I would like to know why the default size is so small.