Add a placeholder image ID setting in WordPress, under Settings > Media

placeholder-image-id-setting.php

<?php
/**
 * Registers any additional settings required for the plugin.
 */

/**
 * Registers the setting for the placeholder attachment ID.
 */
function hd_utility_register_image_placeholder_id_setting() {

	register_setting(
		'media',
		'hd_utility_placeholder_image_id',
	);

}

add_action( 'admin_init', 'hd_utility_register_image_placeholder_id_setting' );

/**
 * Adds the setting to the media page in the WP admin area.
 */
function hd_utility_add_placeholder_id_setting() {

	add_settings_field(
        'srgroup-utility-placeholder-id',
        __( 'Placeholder Image ID', 'srgroup-utility' ),
        'hd_utility_placeholder_id_setting_output',
        'media',
        'default',
        array(
			'label_for' => 'srgroup-placeholder-id',
       )
    );

}

add_action( 'admin_init', 'hd_utility_add_placeholder_id_setting' );

/**
 * Outputs the settings field for the placeholder image ID.
 */
function hd_utility_placeholder_id_setting_output() {

	?>
	<input type="text" name="<?php echo esc_attr( 'hd_utility_placeholder_image_id' ); ?>" id="<?php echo esc_attr( 'hd_utility_placeholder_image_id' ); ?>" class="regular-text" value="<?php echo esc_attr( get_option( 'hd_utility_placeholder_image_id' ) ); ?>" />
	<p class="description"><?php esc_html_e( 'Enter the attachment ID of the image to be used as a placeholder', 'srgroup-utility' ); ?></p>
	<?php

}