Ah, yeah. The “margins” of a prototype also divide up the prototype background into a 3x3 grid. When a contraption is resized: the top-left tile of the prototype is drawn at the top-left corner of the contraption; the top-centre tile of the prototype is tiled horizontally across the top of the contraption; the top-right tile of the prototype is drawn at the top-right corner of the contraption, and so forth.
The default introduction/tutorial deck you get when you launch Decker has a page on “contraptions”, and one of the examples is a “resizable decorative border” contraption that demonstrates the behaviour nicely.
If you want to draw the background with code, you’ll need a canvas widget if only because the Image protocol you get from card.image doesn’t provide a lot of drawing primitives. You can use a “hidden” canvas widget, draw the background you want, then do card.image:canvas.copy[0,0 canvas.lsize] to paste it onto the contraption background, or (much easier) just make the canvas widget cover the area of your prototype, and Decker’s default resize behaviour (with the margins and such) will make sure it stays in place as the contraption is resized.