iTunes new Grid view has pre-made graphics for all of the default genres in iTunes, but it may not cover the way you define your music. Normally, if you have genre that iTunes does not recognize, it will just use cover artwork available from your music library to display that genre. Luckily, if you'd like to create or change any of the genre images it's easy.
In your favorite image editing program, create a 256x256, 72ppi image for the genre of your choice. Save the file as genre-yourgenrename.jpg (example: genre-newwave.jpg). Next, go to the Applications folder, Control-click on iTunes and select Show Package Contents from the pop-up menu. In the new window that opens, navigate to iTunes » Content » Resources. Drag and drop your image into this directory.
To get iTunes to recognize your image, you will also need to edit the genres.plist file in the Resources directory. (As always, back the original up for safety, edit at your own risk, etc.) Now just add in the following string:
<dict>
<key>matchString</key><string>yourgenrename</string>
<key>resourceFile</key><string>genre-genrename.jpg</string>
</dict>
Obviously, replace yourgenrename and genre-genrename.jpg with the appropriate names you use in iTunes and the name of the image you created earlier.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008091401425620