Thank you for your reply.
This works for previously uploaded images as well.
In case you have difficulties with that, please e-mail us at:
http://support.emagicone.com/submit_ticket
and provide us with your store connection configuration (you can save it at Preferences[F12] / Actions / Save current configuration), sku of the product that you are modifying and re-size rules, that you used, so we could check what caused the issue for you.
Waiting to hear from you