Flush all derived images for a given style.
drush image-flush
style : An image style machine name. If not provided, user may choose from a list of names.
--all : Flush all derived images
Pick an image style and then delete its images.
drush image-flush
Delete all thumbnail images.
drush image-flush thumbnail
Flush all derived images. They will be regenerated on the fly.
drush image-flush --all