def writetitle(self, titlestring, colour = None):
        """
        We write a title, centered below the image.
        """
        
        self.checkforpilimage()
        colour = self.defaultcolour(colour)
        self.changecolourmode(colour)
        self.makedraw()
        
        self.loadtitlefont()
        
        imgwidth = self.pilimage.size[0]
        imgheight = self.pilimage.size[1]
        textwidth = self.draw.textsize(titlestring, font = self.titlefont)[0]
        textxpos = imgwidth/2.0 - textwidth/2.0
        textypos = imgheight - 30
        
        self.draw.text((textxpos, textypos), titlestring, fill = colour, font = self.titlefont)
        
        if self.verbose :
            print "I've written a title on the image."