I've searched around a little on the forums and can't seem find any repositories for cover art that would blend nicely with the default channels interface. (Font, Color, Size). The default purple gradient with grey text is pretty good and works well with 3-6 collections, but anything more starts to make things a little harder to find.
It would be great to have a library of public domain art work to choose from, at least for the generic type collections/playlists like news, sports, series, biography, specials, holidays, ....
If I had an ounce of artistic ability, this might be an easier endeavor.