patch from comment #15 applied with heavy modification...

to implement an action your module need to implement hook_imagecache_actions (see php doc in
imagecache implementation).

actions can exist inside your module or in an external include
in the same directory as the implementing module.

there are two call backs for each action implemented...
$action['action'] .'_form($data)' -- preset form....
$action['action'] .'_image($image, $data)' -- this callback does the work...