Customizing The Header Background Image in Astra Theme For WordPress

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("");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("");background-repeat: no-repeat;