I am not sure if is easy but, you probably want to add a gallery field to the promotions, and modify the “add_meta_box” for the promotions area “ims_new_promotion” (admin/pricing.php).
and to check the promotion you need to add the validation on the cart update function hook “ims_before_update_cart” or “ims_after_update_cart”. file (_inc/store.php)
hope this helps