I can actually answer that here because it does tie to iTunes. The iPad PROBABLY (Im assuming) shares the same genre artwork set that iTunes does. Here is a list of what iTunes (and probably your iPad) have:
http://farm4.static.flickr.com/3296/2845782938_70c5872fce.jpg
Any other genre will be listed with the music note as it does not have artwork for it.
This article explains how it works:
http://blogs.oreilly.com/digitalmedia/2008/09/custom-genre-artwork-for-itune.htm l
It's apparently in your iTunes Application System Files in /Applications/iTunes/Contents/Resources.
Apparently some freeware software will let you assign artwork to other genres if you have any and wish to have this try some software like this:
http://genreman.sourceforge.net/ or do a google search for Change iTunes Genre Artwork. However, remember this is all 3rd party software and isnt officially supported by Apple.
This however, may NOT carry over onto the iPad. Im not sure, you'd have to see for yourself. My guess is that it wouldn't.
