In a skin, you can override the genre icon (id=UnknownGenre) but its only used in the thumbnail browser
In the left navigator, the icon fort the music explorer node can be overridden (id=MusicExplorerIcon). It just so happens the default icon I have looks the same as the genre icon.
In the music explorer, when viewing groupings such as genre, the icon you see is always used. There is no current override for it
In the music explorer, when viewing an artist with no picture, then unknown artist icon is used (override id=UnknownArtist). It is displayed at 125x125px