How to render random backgrounds with HTML5 Scratch Card

I am using HTML5 Scratch Card plugin and dont know how I can render random backgrounds with javascript.Here is the code

window.onload = function() {
            createScratchCard({
                'container':document.getElementById('circle'),
                'background':'assets/images/demo2-circle-background.png',
                'foreground':'assets/images/demo2-circle-foreground.png',
                'percent':100,
                'coin':'assets/images/coin2.png',
                'thickness':18,
            });
        }

So with this default structure how I can render random 2, 3 images and show one every time, show randomly>