Mina-san,
Recently we started tagging lessons with Album Artwork that is viewable in iTunes, your iPod, Windows Media Player or any media player that supports Album Artwork. If you've already downloaded previous lessons, the artwork probably won't update automatically without a new download.
If you'd like to apply the artwork manually, it's really easy to do in iTunes!
1) Download the artwork to your desktop.
http://www.japanesepod101.com/images/itunes_logo.jpg
2) In your "Podcasts" directory, press CTRL-A (on Mac press ⌘-A) to select ALL.
3) Right click and select "Get Info" (on Mac press ⌘-I).
4) Drag the album artwork from the desktop into the Album Artwork Box.
5) Click OK.
Depending on your computer's speed, it may take a few minutes to update all the lessons.