The background image of a card is "card.image", an Image Interface. You can do some kinds of drawing and compositing directly with an Image, but it is less general than a Canvas Interface. Images are simple values which can be created on the fly, while Canvases are widgets that are attached to a deck and have awareness of e.g. the deck's palette, brushes, and fonts.
You could make a canvas the size of an entire card, if you like, and/or draw to a canvas elsewhere and use .copy[] and .paste[] to transfer image data to the card background.