Yes, it can take some time, typically 24-48 hours...unless you manually Ping iTunes. Here's how to do it (this is from the iTunes spec page at
http://www.apple.com/itunes/store/podcaststechspecs.html
You can ping the iTunes server to update your podcast information by entering the following URL into a browser:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/pingPodcast?id=FEEDID
where FEEDID is your iTunes podcast numeric ID. You can also use:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/pingPodcast?feedURL=http://FEEDURL
where FEEDURL is the url for your feed.
Regardless of the ping method you use, iTunes will return an HTTP 200 code.
Jon again,
If you Ping, iTunes should update within a few minutes to a few hours. And you'll see your artwork if everything is correct!
Jon
TreeLine Productions
Fort Collins, CO USA
Currently producing these popular podcasts:
"The Rest of Everest" Video Podcast *Voted as one of iTunes' "Best of 2006" podcasts
*Nominated for "Best Video Podcast" in the 2007 People's Choice Podcast Awards
www.therestofeverest.com
"Beyond the Movement: The Pilates Podcast"
www.thepilatespodcast.com