wp_create_thumbnail │ filter-hook │ WP 1.0

Usage

add_filter( 'wp_create_thumbnail', 'wp_kama_create_thumbnail_filter' );

/**
 * Function for `wp_create_thumbnail` filter-hook.
 * 
 * @param  $image_resize 
 *
 * @return 
 */
function wp_kama_create_thumbnail_filter( $image_resize ){

	// filter...
	return $image_resize;
}

$image_resize

-

Where the hook is called

return apply_filters( 'wp_create_thumbnail', image_resize( $file, $max_side, $max_side ) );

Where the hook is used in WordPress