When viewing your podcast listing on iTunes, it shows the artwork from your feed. When viewing a specific episode, it shows the artwork within that episode media file.
I downloaded TNM118-MarkMichaelLewis.mp3, opened it in iTunes and confirmed iTunes does not display your artwork. If I go to "get info" I do see the podcast artwork but it's very small, not even 300x300. Try using your latest 1400x1400 artwork, as that is now what iTunes wants, then see if it appears in iTunes.