WordPress at a glance
function is not described

_wp_post_thumbnail_class_filter() WP 2.9.0

Adds a 'wp-post-image' class to post thumbnails. Internal use only.

Uses the 'begin_fetch_post_thumbnail_html' and 'end_fetch_post_thumbnail_html' action hooks to dynamically add/remove itself so as to only filter post thumbnails.

No Hooks.

Return

Array. Modified array of attributes including the new 'wp-post-image' class.

Usage

_wp_post_thumbnail_class_filter( $attr );
$attr(array) (required)
Thumbnail attributes including src, class, alt, title.

Code of wp post thumbnail class filter: wp-includes/media.php VER 5.1.1

<?php
function _wp_post_thumbnail_class_filter( $attr ) {
	$attr['class'] .= ' wp-post-image';
	return $attr;
}