Assert if image exists
        Args:
            - pattern: image filename # not support pattern for now
            - timeout (float): seconds
            - safe (bool): not raise assert error even throung failed.

def assert_image_exists(self, pattern, timeout=20.0, **kwargs):
        """
        Assert if image exists
        Args:
            - pattern: image filename # not support pattern for now
            - timeout (float): seconds
            - safe (bool): not raise assert error even throung failed.
        """
        pattern = self.d.pattern_open(pattern)
        match_kwargs = kwargs.copy()
        match_kwargs.pop('safe', None)
        match_kwargs.update({
            'timeout': timeout,
            'safe': True,
        })
        res = self.d.wait(pattern, **match_kwargs)
        is_success = res is not None
        message = 'assert image exists'
        if res:
            x, y = res.pos
            kwargs['position'] = {'x': x, 'y': y}
            message = 'image exists\npos %s\nconfidence=%.2f\nmethod=%s' % (res.pos, res.confidence, res.method)
        else:
            res = self.d.match(pattern)
            if res is None:
                message = 'Image not found'
            else:
                th = kwargs.get('threshold') or pattern.threshold or self.image_match_threshold
                message = 'Matched: %s\nPosition: %s\nConfidence: %.2f\nThreshold: %.2f' % (
                    res.matched, res.pos, res.confidence, th)

        kwargs['target'] = self._save_screenshot(pattern, name_prefix='target')
        kwargs['screenshot'] = self.last_screenshot
        kwargs.update({
            'action': 'assert_image_exists',
            'message': message,
            'success': is_success,
        })
        self._add_assert(**kwargs)