public function addFrame($data = null)
    {
        $frame = new Frame();

        if (gettype($data) == 'object' && get_class($data) == 'movemegif\domain\FileImageCanvas')
            $frame->setCanvas($data);
        else if (is_string($data) && file_exists($data))
            $frame->setCanvas(new FileImageCanvas($data));
        else if ($data != null)
            throw new MovemegifException('Unknown datatype. Use either a path or a Canvas object.');

        $this->extensions[] = $frame;
        return $frame;
    }