If your image is indeed in the feed, in the itunes artwork tag and embedded in the mp3 file, you will have to contact Apple and ask them what the issue is.
They have been really good at answering questions about iTunes listings. Just make sure you double check that the image is in all 3 places. (RSS image, iTunes image and in your iD3 tags) . iTunes podcasting support is podcasts@apple.com
-Mike