def _pngmeta(self):
        """It will return GeoImage.tags as a PNG metadata object.

        Inspired by:
        public domain, Nick Galbreath
        http://blog.modp.com/2007/08/python-pil-and-png-metadata-take-2.html
        """
        reserved = ('interlace', 'gamma', 'dpi', 'transparency', 'aspect')

        try:
            tags = self.tags
        except AttributeError:
            tags = {}

        # Undocumented class
        from PIL import PngImagePlugin
        meta = PngImagePlugin.PngInfo()

        # Copy from tags to new dict
        for k__, v__ in tags.items():
            if k__ not in reserved:
                meta.add_text(k__, v__, 0)

        return meta