Hello @gawra,
Please try to use the following CSS code.
You can add CSS code in Dashboard → Appearance → Customize → Additional CSS (WordPress 4.7 and up). Also you can add CSS code directly to style.css
file of your child theme.
header#masthead.float-header {
background-image: url('https://example.com/example.jpg');
background-size: cover;
background-repeat: no-repeat;
}
Kind Regards, Roman.