Blog header background image is ugly on mobile size

My problem is simple : when i join an image with an article the result is good on desktop and tablet size but ugly on mobile.
A magic CSS to fix that?
Thank you in advance to any one who may be able to give me some ideas.

Hello there,

I hope you are doing well today.

You can use the following CSS code to edit the image properties by going to Appearance > Customize > Additional CSS and pasting it there.

@media only screen and (max-width: 1024px)
#header {
    height: 400px;
    width: auto;

Best Regards,