It’s not in the index.php. It’s in wp-includes/post-thumbnail-template.php (I found searching for it in a downloaded copy):
<?php
/**
* WordPress Post Thumbnail Template Functions.
*
* Support for post thumbnails
* Themes function.php must call add_theme_support( 'post-thumbnails' ) to use these.
*
* @package WordPress
* @subpackage Template
*/
/**
* Check if post has an image attached.
*
* @since 2.9.0
*
* @param int $post_id Optional. Post ID.
* @return bool Whether post has an image attached.
*/
function has_post_thumbnail( $post_id = null ) {
return (bool) get_post_thumbnail_id( $post_id );
}
/**
* Retrieve Post Thumbnail ID.
*
* @since 2.9.0
*
* @param int $post_id Optional. Post ID.
* @return int
*/
function get_post_thumbnail_id( $post_id = null ) {
$post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
return get_post_meta( $post_id, '_thumbnail_id', true );
}
/**
* Display Post Thumbnail.
*
* @since 2.9.0
*
* @param string|array $size Optional. Image size. Defaults to 'post-thumbnail', which theme sets using set_post_thumbnail_size( $width, $height, $crop_flag );.
* @param string|array $attr Optional. Query string or array of attributes.
*/
function the_post_thumbnail( $size = 'post-thumbnail', $attr = '' ) {
echo get_the_post_thumbnail( null, $size, $attr );
}
/**
* Update cache for thumbnails in the current loop
*
* @since 3.2.0
*
* @param object $wp_query Optional. A WP_Query instance. Defaults to the $wp_query global.
*/
function update_post_thumbnail_cache( $wp_query = null ) {
if ( ! $wp_query )
$wp_query = $GLOBALS['wp_query'];
if ( $wp_query->thumbnails_cached )
return;
$thumb_ids = array();
foreach ( $wp_query->posts as $post ) {
if ( $id = get_post_thumbnail_id( $post->ID ) )
$thumb_ids[] = $id;
}
if ( ! empty ( $thumb_ids ) ) {
_prime_post_caches( $thumb_ids, false, true );
}
$wp_query->thumbnails_cached = true;
}
/**
* Retrieve Post Thumbnail.
*
* @since 2.9.0
*
* @param int $post_id Optional. Post ID.
* @param string $size Optional. Image size. Defaults to 'post-thumbnail'.
* @param string|array $attr Optional. Query string or array of attributes.
*/
function get_the_post_thumbnail( $post_id = null, $size = 'post-thumbnail', $attr = '' ) {
$post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
$post_thumbnail_id = get_post_thumbnail_id( $post_id );
/**
* Filter the post thumbnail size.
*
* @since 2.9.0
*
* @param string $size The post thumbnail size.
*/
$size = apply_filters( 'post_thumbnail_size', $size );
if ( $post_thumbnail_id ) {
/**
* Fires before fetching the post thumbnail HTML.
*
* Provides "just in time" filtering of all filters in wp_get_attachment_image().
*
* @since 2.9.0
*
* @param string $post_id The post ID.
* @param string $post_thumbnail_id The post thumbnail ID.
* @param string $size The post thumbnail size.
*/
do_action( 'begin_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size );
if ( in_the_loop() )
update_post_thumbnail_cache();
$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );
/**
* Fires after fetching the post thumbnail HTML.
*
* @since 2.9.0
*
* @param string $post_id The post ID.
* @param string $post_thumbnail_id The post thumbnail ID.
* @param string $size The post thumbnail size.
*/
do_action( 'end_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size );
} else {
$html = '';
}
/**
* Filter the post thumbnail HTML.
*
* @since 2.9.0
*
* @param string $html The post thumbnail HTML.
* @param string $post_id The post ID.
* @param string $post_thumbnail_id The post thumbnail ID.
* @param string $size The post thumbnail size.
* @param string $attr Query string of attributes.
*/
return apply_filters( 'post_thumbnail_html', $html, $post_id, $post_thumbnail_id, $size, $attr );
}
