The Store is showing a different version of the image from that in the feed - in the feed there is a black surround, but not in the Store.


I assume the image on the right was previously in the feed; it's been cached by the Store. It won't be replaced by the image in the feed, because it's 1005 x 909 px; it must be minimum 1400 x 1400.
When you subscribe, the image you see when playing an episode is not derived from the feed, but has to be actually embedded in the media file. You don't have this; I would have expected to see the generic iTunes logo, as below, rather than black, so I don't know why you're seeing that.

For a detailed explanation of the embedded image, please see http://wilmut.uk/coverart