ImageCache Scale-9 Actions provides one way to add illustrated corners, borders and (manually created) dropshadows.
ImageCache Scale-9 Actions was Drupal 6 only, and looks like it has no active upgrade support coming. We should be able to take this on pretty easily and adopt it in imagecache_actions.
Greggles has expressed an interest.
Setting this issue up for scale9 specifically to distinguish it from earlier 'dropshadow' discussions #293238: Drop Shadow Effect