Lines:

<?php
/* function jetpack_featured_images_fallback_get_image() – Get one image from a specified post in the following order:
Featured Image then first image from the_content HTML
and filter the post_thumbnail_html */
add_filter( 'post_thumbnail_html', 'jetpack_featured_images_fallback_get_image', 10, 5 );

/* function jetpack_featured_images_fallback_get_image_src() – Get URL of one image from a specified post in the following order:
Featured Image then first image from the_content HTML */

/* function jetpack_has_featured_image() – Check if post has an image attached, including a fallback. */

/* function jetpack_featured_images_post_class() – Adds custom class to the array of post classes. */
add_filter( 'post_class', 'jetpack_featured_images_post_class', 10, 3 );
 

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