Respond to image style flushing.
This hook enables modules to take effect when a style is being flushed (all images are being deleted from the server and regenerated). Any module-specific caches that contain information related to the style should be cleared using this hook. This hook is called whenever a style is updated, deleted, or any effect associated with the style is update or deleted.
$style: The image style array that is being flushed.
function hook_image_style_flush($style) {
// Empty cached data that contains information about the style.
cache_clear_all('*', 'cache_mymodule', TRUE);
}