def upload_image(self, media_file):
res = self._post(<EOL>url='<STR_LIT>',<EOL>files={<EOL>'<STR_LIT>': media_file<EOL>},<EOL>result_processor=lambda x: x['<STR_LIT:url>']<EOL>)<EOL>return res<EOL>
上传群发消息内的图片
详情请参考
http://mp.weixin.qq.com/wiki/15/5380a4e6f02f2ffdc7981a8ed7a40753.html

:param media_file: 要上传的文件，一个 File-object
:return: 上传成功时返回图片 URL