Add ability to expire and remove images of certain presets after a given period of time

It would be useful to be able to set a preset so that after a given expiry time, an image within the cache is expired. Perhaps as a configurable drupal cron hook that would sift through images of given presets and remove those that have expired.

A use case for this would be caching of website screenshots, which should be refreshed regularly, but the existing flush function would remove all images within a preset, resulting in slowdowns.

Currently one way this can be done is with a server-side cron job which selectively removes files within imagecache folders, but it would be good to see this as part of this module or as a submodule?