Editing the file index.html should do the trick.
!-- All margins and padding must be zero for the canvas to fill the screen. -->
<style type="text/css">
* {
padding: 0;
margin: 0;
}
body {
<font color="red">background: #000;</font>
color: #fff;
overflow: hidden;
-ms-touch-action: none;
}
canvas {
?touch-action-delay: none;
?touch-action: none;
?-ms-touch-action: none;
}
</style>
Replace <font color="red">background: #000</font> with <font color="blue">background-image:url('your_image.png')</font> .
More info here: http://www.w3schools.com/css/css_background.asp