Hook into custom image_size?

Hey everyone!

In my plugin i create a additional image size ala:

add_action('init', 'reg_appthumb');

function reg_appthumb() {
	add_theme_support( 'post-thumbnails' );
	add_image_size( 'app-thumb', pxwide, pxheight );
}

For our app-users we need smooth thumbs with rounded corners and a dropshadow (without CSS3!). So i created a small imagick-function for an advanced image manipulation and it works well.

My Question: how can i trigger this function, whenever the app-thumb is created? I think a gonna hook me into the add_attachment, right? Any ideas?

Thanks alot!