'Set background image or return name of current backgroundimage.
Optional argument:
picname -- a string, name of a gif-file or "nopic".
If picname is a filename, set the corresponding image as background.
If picname is "nopic", delete backgroundimage, if present.
If picname is None, return the filename of the current backgroundimage.
Example (for a TurtleScreen instance named screen):
>>> screen.bgpic()
\'nopic\'
>>> screen.bgpic("landscape.gif")
>>> screen.bgpic()
\'landscape.gif\''

def bgpic(self, picname=None):

    if (picname is None):
        return self._bgpicname
    if (picname not in self._bgpics):
        self._bgpics[picname] = self._image(picname)
    self._setbgpic(self._bgpic, self._bgpics[picname])
    self._bgpicname = picname
