Hi James,
Probably the easiest way to do this is to use the custom fields to store the filenames of the other images. There are 5 custom fields on product for you to use as you wish.
These custom fields are returned/updated by the KonaKart APIs just as the others are, so you use them in the same way as the other attributes in the app. Therefore I would look at the way the app uses prod.getImage() and make the necessary changes to use prod.getCustom1() or whatever is appropriate for your needs.
(There are similar custom attributes available on other objects such as customer, order, manufacturer etc. They can all be maintained via the KonaKart Admin APIS - and also by using the KonaKart admin App).
Good luck,
Trevor