Unfortunately, you have to include [dpsc_display_product] in every post unless you create a custom single-duka.php that includes it by default. This file is your theme template file that handles products. It is basically a custom single.php that gets called for only products
Unfortunately at this time there is no other way but to use timthumb
DukaPress is open source software that can be used to build online shops quickly and easily. DukaPress is built on top of Wordpress, a world class content management system. DukaPress is built to be both simple and elegant yet powerful and scalable.