acf-options.php

<?php
function cameronjonesweb_acf_setup() {
	acf_add_options_page(
		array(
			'page_title' => 'Theme Settings',
			'menu_slug'  => 'cameronjonesweb-theme-settings',
		)
	);
	acf_add_local_field_group(
		array(
			'key'    => 'group_cameronjonesweb_theme_settings',
			'title'  => 'Theme Settings',
			'fields' => array(
				array(
					'key' => 'field_cameronjonesweb_default_image',
					'label' => 'Default Featured Image',
					'name' => 'default_featured_image',
					'type' => 'image',
				),
			),
			'location' => array (
				array (
					array (
						'param' => 'options_page',
						'operator' => '==',
						'value' => 'cameronjonesweb-theme-settings',
					),
				),
			),
		)
	);
}

add_action( 'acf/init', 'cameronjonesweb_acf_setup' );

default-featured-image.php

<?php
/**
 * @link https://wordpress.stackexchange.com/a/175179/65582
 * @return null|int Null if the post already has a featured image, ID of the default image if not
 */
function cameronjonesweb_maybe_use_default_featured_image( $value, $post_id, $meta_key, $single ) {
	// Only run on the front end.
	if ( ! is_admin() && ! wp_is_json_request() ) {
		// Only apply to specific post types.
		if ( in_array( get_post_type( $post_id ), array( 'post' ) ) ) {
			$meta_needed = '_thumbnail_id';
			// Only run specifically on the thumbnail meta.
			if ( isset( $meta_key ) && $meta_needed === $meta_key ) {
				// We need to get the meta without this function running, so we remove it all inception like.
				remove_filter( 'get_post_metadata', 'cameronjonesweb_maybe_use_default_featured_image', 100 );
				// This will be the ID of the thumbnail if the post has one already.
				$current_meta = get_post_meta( $post_id, $meta_needed, true );
				// Now that we have the normal value, add the function back.
				add_filter( 'get_post_metadata', 'cameronjonesweb_maybe_use_default_featured_image', 100, 4 );
				// If it doesn't have a thumbnail set, inject our default image.
				$value = ! empty( $current_meta ) ? $current_meta : get_field( 'default_featured_image', 'options', false );
			}
		}
	}
	return $value;
}

add_filter( 'get_post_metadata', 'cameronjonesweb_maybe_use_default_featured_image', 100, 4 );