Astra theme for WordPress, at least the free version does not allow background images in the header through the customizer. In order to add a background image to the header you will need to add some css. Go to Appearance/Customize/Additional CSS.
.main-header-bar {background-image: url("http://yoursite.com/wp-content/uploads/2017/10/your-header-logo.png");background-repeat: no-repeat;
}
Here’s a bit of CSS that addds a drop shadow and background color to the header..main-header-bar {background: #FFBC00;box-shadow: 0px 3px 3px #b8b8b8;background-image: url("http://yoursite.com/wp-content/uploads/2017/10/your-header-logo.png");background-repeat: no-repeat;
}