Or maybe you could set the window transparant and add an Image?
in your html body, specify the alpha of the background colour, in my example below, I set it to be full transparent
you can set the window's transparency "on the fly" using
var gui = require('nw.gui');
var win = gui.Window.get();
you can also specify it in package.json
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..