The 512 by 342 size of a Card includes the space under the Menu bar. You can hide the Menu bar with the "M" key on your keyboard while you're placing your imported images (or otherwise using drawing tools) on your cards in Decker.
To explain a little more.... if you choose to 'Lock'/'Protect' a deck for publication the Menu bar and all of Decker's editing tools are hidden from your users and that space underneath the Menu bar becomes visible on every card.
Beautiful art, by the way!