Backstretch Image to CSS Background Image in Agent Focused Theme - https://amethystwebsitedesign.com/backstretch-image-to-css-background-image-in-agent-focused-theme/

front-page.php

<?php // Do not add this line when you copy and paste code.

// Enqueue scripts for background image.
add_action( 'wp_enqueue_scripts', 'agentfocused_front_page_enqueue_scripts' );
function agentfocused_front_page_enqueue_scripts() {

	wp_enqueue_style( 'agentfocused-front-page-styles', get_stylesheet_directory_uri() . '/style-front.css', array(), CHILD_THEME_VERSION );

/*
	$image = get_option( 'agentfocused-front-image', sprintf( '%s/images/front-page-image.jpg', get_stylesheet_directory_uri() ) );

	// Load scripts only if custom background is being used.
	if ( ! empty( $image ) && is_active_sidebar( 'front-page-1' ) ) {

		// Enqueue Backstretch scripts.
		wp_enqueue_script( 'agentfocused-backstretch', get_stylesheet_directory_uri() . '/js/backstretch.js', array( 'jquery' ), '1.0.0' );

		wp_enqueue_script( 'agentfocused-backstretch-set', get_stylesheet_directory_uri() . '/js/backstretch-set.js' , array( 'jquery', 'agentfocused-backstretch' ), '1.0.0' );

		wp_localize_script( 'agentfocused-backstretch-set', 'BackStretchImg', array( 'src' => str_replace( 'http:', '', $image ) ) );

	}
*/

	// Enqueue Modernizr for object-fit.
	wp_enqueue_script( 'agentfocused-modernizr-object-fit', get_stylesheet_directory_uri() . '/js/modernizr-custom.js', array( 'jquery' ), '1.0.0' );
}

// Add background image style to WP head.
add_action( 'wp_head', 'agentfocused_background_styles', 99 );
function agentfocused_background_styles() {

	$image = get_option( 'agentfocused-front-image', sprintf( '%s/images/front-page-image.jpg', get_stylesheet_directory_uri() ) );

	// Add style only if background image is being used.
	if ( ! empty( $image ) && is_active_sidebar( 'front-page-1' ) ) {

		echo '<style type="text/css" media="screen">
			body.front-page {
				background-image: url(' . esc_html( $image ) . ');
			}
		</style>';

	}
}

Notes.txt

The Steps to Take

1. First, we’ll comment out the sections that add the backstretch script.
2. Next we’ll add a new function to add the body background-image style, both in front-page.php.
3. And then we’ll add the positioning styles to style-front.css.

style-front.css

body.front-page {
    background-attachment: fixed;
    background-position: center center;
    background-repeat: no-repeat;
    -webkit-background-size: cover;
    -moz-background-size:    cover;
    background-size:         cover;
}