WordPress at a glance
action is not described

begin_fetch_post_thumbnail_html action-hook . WP 2.9.0

Fires before fetching the post thumbnail HTML.

Provides "just in time" filtering of all filters in wp_get_attachment_image().


add_action( 'begin_fetch_post_thumbnail_html', 'action_function_name_3057', 10, 3 );
function action_function_name_3057( $post_id, $post_thumbnail_id, $size ){
	// action...
The post ID.
The post thumbnail ID.
The post thumbnail size. Image size or array of width and height values (in that order).
Default: 'post-thumbnail'


Since 2.9.0 Introduced.

Where the hook is called

wp-includes/post-thumbnail-template.php 175
do_action( 'begin_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );

Where in WP core the hook is used WordPress

wp-includes/post-thumbnail-template.php 389
add_action( 'begin_fetch_post_thumbnail_html', '_wp_post_thumbnail_class_filter_add' );