Well, without your posting the URLs of your iTunes Store page and feed one could only indulge in wild guesswork. However, you should be aware that the image placed within episode tags does not appear in iTunes when subscribing nor in the Store (except possibly very small in the Search results, though I'm not sure whether this still applies). Only the image in the top section of the feed, before the first 'item' tag, appears on the Store page. Images which appear when subscribed in iTunes and playing an episode have to be actually embedded in the media file - see http://wilmut.uk/coverart