This?
https://github.com/nwjs/nw.js/issues/811
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
<body style="background-color:rgba(0,0,0,0);">
you can set the window's transparency "on the fly" using
var gui = require('nw.gui');
var win = gui.Window.get();
win.setTransparent(!win.isTransparent);
you can also specify it in package.json
"window": {
"transparent": true
}