Purging of images is now in CVS.
For scaled images, this depends on visiting the page containing the scaled
image once without caching, which means it is automatic for users with
session.
The image is purged whenever a new scaled image is generated which is
done whenever a link to this image is rendered. In some cases anon users
might still have to reload to get the new scaled image, but this should be
rare.
Unscaled images are purged immediately on edit, deletion or revert.
--
Gabriel Wicke