Custom “Insert into Post” button

by

If you want it to show up in the image edit, say next to “insert into post” you will need to hook into 

attachment_fields_to_edit 
//for instance
add_filter( 'attachment_fields_to_edit', 'yourfunction', 1, 2 );

Have a look at media.php
http://mobile.adambrown.info/p/wp_hooks/hook/attachment_fields_to_edit?version=3.1&file=wp-admin/includes/media.php