Drupal 7 - Removes all the style images generated so new ones can be created when needed.

flush_style_images.php

<?php
module_load_include('module', 'image');
$styles = array_keys(image_styles());
image_style_flush($styles);

drupal_set_message(t('Flushed images for the following styles: %styles.', array('%styles' => implode(', ', $styles))), 'status', FALSE);