Resize all thumbs in WP

wp_reset_thumbs_sizes.php

<?php 

/** 
 * Resize all thumbs in WP when thumbnail settings are updated
 */
function update_thumbnails() {
	
	set_time_limit(0); // can help if lot of images
	
	$attachments = get_children( array('post_type' => 'attachment', 'post_mime_type' => 'image') );

	if ( !$attachments )
		return;

	foreach ( $attachments as $id => $attachment ) {
		$imagedata = wp_get_attachment_metadata($id);
		$thumbnail = dirname($imagedata['file']) . '/' . $imagedata['sizes']['thumbnail']['file'];

		if ( file_exists($thumbnail) )
			@unlink($thumbnail);

		wp_update_attachment_metadata($id, wp_generate_attachment_metadata($id, $imagedata['file']));
	}
}
add_action('update_option_thumbnail_size_w', 'update_thumbnails');
add_action('update_option_thumbnail_size_h', 'update_thumbnails');
add_action('update_option_thumbnail_crop', 'update_thumbnails');