How Do You Change The Default Genre Artwork in iTunes?
Doing it manually:
You can also place your genre image in /Applications/iTunes.app/Contents/Resources (shift+cmd+G and paste this path to open it directly) and add your genre in genres.plist file. Use TextEdit or your favourite Editor of that kind (I use TextWrangler).
Copy and paste an existing list object like this one:

&lt;dict&gt;
    &lt;key&gt;matchString&lt;/key&gt;&lt;string&gt;blues&lt;/string&gt;
    &lt;key&gt;resourceFile&lt;/key&gt;&lt;string&gt;genre-blues.jpg&lt;/string&gt;
&lt;/dict&gt;


Then modify it so it contains your new genre and image like this one:

&lt;dict&gt;
    &lt;key&gt;matchString&lt;/key&gt;&lt;string&gt;Psychedelic Rock&lt;/string&gt;
    &lt;key&gt;resourceFile&lt;/key&gt;&lt;string&gt;genre-psychedelic.jpg&lt;/string&gt;
&lt;/dict&gt;


It's best if you copy the existing .plist file to your desktop, edit it and then put the new version back into the source folder and type in your password since it won't let you save your file directly into the folder (no rights in that folder for applications, only Finder via admin rights)
I hope this helps. (I know about the detailed version over at O'Reilly, this is basically the same in short)