微信公众号图文消息群发 显示首图 API问题

在上传了图文消息之后，我使用测试号预览图文消息接口，得到的结果如下:

我想得到的结果如下:

我该如何实现首篇文章是大图的效果？

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附：公众号图文消息API

{    "articles": [     {                         "thumb_media_id":"qI6_Ze_6PtV7svjolgs-rN6stStuHIjs9_DidOHaj0Q-mwvBelOXCFZiq2OsIU-p",                         "author":"xxx",                                "title":"Happy Day",                                 "content_source_url":"www.qq.com",                                "content":"content",                                 "digest":"digest",                         "show_cover_pic":1,                         "need_open_comment":1,                         "only_fans_can_comment":1                         },                             {                         "thumb_media_id":"qI6_Ze_6PtV7svjolgs-rN6stStuHIjs9_DidOHaj0Q-mwvBelOXCFZiq2OsIU-p",                         "author":"xxx",                                "title":"Happy Day",                                 "content_source_url":"www.qq.com",                                "content":"content",                                 "digest":"digest",                         "show_cover_pic":0,                         "need_open_comment":1,                         "only_fans_can_comment":1                         }    ] }