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As you say the image has not updated: I can't tell when you uploaded this (the 'last build date is 1518 GMT and so later than your post). It can take several days for a changed image to appear in the Store, so it's probably just a question of waiting, provided that you gave the new image a different file name from the old - this is vital because the Store caches the image and if you don't change its URL the Store has no way of knowing it's changed.
For reference, your Store page is at https://podcasts.apple.com/podcast/勞碌命取暖會/id1501529554