Exported images and editor state files can easily be watermarked. Simply specified the text you want via watermarkText option. Now when user exports or downloads a photo, it will be automatically watermarked with specified text.

var pixie = new Pixie({
    watermarkText: 'My Watermark'
});

Photos can also be watermarked manually via watermarkTool.

Note:
Adding watermark this way will make it visible in the editor as well and not just in downloaded or exported photo.
var pixie = new Pixie({
    onLoad: function() {
        pixie.getTool('watermark').add('My Watermark');
    }
});