I'm not sure what a missing .png has to do with the missing album- and genrethumbs. Did you try to refresh the Album/Artist Info through the Context Menu (F9)?In all skins "previewbackground.png" is missing, so no new album- and genrethumbs are created.
2012-12-23 20:22:18.920064 [Debug][MPMain(1)]: Window: MediaPortal.GUI.Music.GUIMusicFiles init
2012-12-23 20:22:18.930064 [Info.][MPMain(1)]: open folderdatabase
2012-12-23 20:22:19.020069 [Debug][MPMain(1)]: Window: MediaPortal.GUI.Music.GUIMusicFiles deinit
2012-12-23 20:22:19.025070 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:19.037070 [Warn.][MPMain(1)]: GUIWindow:OnWindowLoaded: 'C:\ProgramData\Team MediaPortal\MediaPortal\Skin\Titan\mymusicgenres.xml' is missing control id 8 (window property: btnSearch)
2012-12-23 20:22:19.037070 [Warn.][MPMain(1)]: GUIWindow:OnWindowLoaded: 'C:\ProgramData\Team MediaPortal\MediaPortal\Skin\Titan\mymusicgenres.xml' is missing control id 12 (window property: btnPlayCd)
2012-12-23 20:22:19.202080 [Debug][MPMain(1)]: Window: MediaPortal.GUI.Music.GUIMusicGenres init
2012-12-23 20:22:19.218081 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select * from albumartist order by strAlbumArtist asc, Filter: albumartist
2012-12-23 20:22:23.267312 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select distinct * from tracks where strAlbumArtist like '%| Ed Sheeran |%' group by strAlbum, strAlbumArtist order by iYear asc, Filter: album
2012-12-23 20:22:23.274313 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:25.393434 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select distinct * from tracks where strAlbumArtist like '%| Ed Sheeran |%' and strAlbum like '+' and strAlbumArtist like '%| Ed Sheeran |%' group by strAlbum, strAlbumArtist, iDisc order by iDisc asc, Filter: tracks
2012-12-23 20:22:25.394434 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select * from tracks where strAlbumArtist like '%| Ed Sheeran |%' and strAlbum like '+' and iDisc like '0' and strAlbumArtist like '%| Ed Sheeran |%' order by iDisc asc, iTrack asc, Filter: tracks
2012-12-23 20:22:25.397434 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:27.797572 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select distinct * from tracks where strAlbumArtist like '%| Ed Sheeran |%' and strAlbum like '+' and strAlbumArtist like '%| Ed Sheeran |%' group by strAlbum, strAlbumArtist, iDisc order by iDisc asc, Filter: tracks
2012-12-23 20:22:27.798572 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select distinct * from tracks where strAlbumArtist like '%| Ed Sheeran |%' group by strAlbum, strAlbumArtist order by iYear asc, Filter: album
2012-12-23 20:22:27.800572 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:28.652620 [Info.][MPMain(1)]: TvNotify:LoadNotifies
2012-12-23 20:22:28.762627 [Info.][MPMain(1)]: TvNotify: 0 notifies
2012-12-23 20:22:29.189651 [Debug][MPMain(1)]: MusicDatabase: GetSongsByFilter - SQL: select * from albumartist order by strAlbumArtist asc, Filter: albumartist
2012-12-23 20:22:29.190651 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:31.777799 [Warn.][MPMain(1)]: GUIWindow:OnWindowLoaded: 'C:\ProgramData\Team MediaPortal\MediaPortal\Skin\Titan\DialogMenu.xml' is missing control id 2 (window property: btnClose)
2012-12-23 20:22:31.778799 [Warn.][MPMain(1)]: GUIWindow:OnWindowLoaded: 'C:\ProgramData\Team MediaPortal\MediaPortal\Skin\Titan\DialogMenu.xml' is missing control id 5 (window property: lblHeading2)
2012-12-23 20:22:31.956809 [Debug][MPMain(1)]: WindowManager: route MediaPortal.GUI.Music.GUIMusicGenres:504->MediaPortal.Dialogs.GUIDialogMenu:2012
2012-12-23 20:22:31.957809 [Debug][MPMain(1)]: DialogWindow: MediaPortal.Dialogs.GUIDialogMenu init
2012-12-23 20:22:33.535900 [Debug][MPMain(1)]: WindowManager: unroute to MediaPortal.Dialogs.GUIDialogMenu:2012->MediaPortal.GUI.Music.GUIMusicGenres:504
2012-12-23 20:22:33.566901 [Debug][MPMain(1)]: Looking up Artist:
2012-12-23 20:22:33.574902 [Warn.][MPMain(1)]: GUIWindow:OnWindowLoaded: 'C:\ProgramData\Team MediaPortal\MediaPortal\Skin\Titan\dialogOK.xml' is missing control id 11 (window property: btnYes)
2012-12-23 20:22:33.583902 [Debug][MPMain(1)]: WindowManager: route MediaPortal.GUI.Music.GUIMusicGenres:504->MediaPortal.Dialogs.GUIDialogProgress:101
2012-12-23 20:22:33.583902 [Debug][MPMain(1)]: DialogWindow: MediaPortal.Dialogs.GUIDialogProgress init
2012-12-23 20:22:34.074931 [Debug][MPMain(1)]: AllmusicScraper. Searching-Artist: Ed Sheeran
2012-12-23 20:22:35.893035 [Debug][MPMain(1)]: AllmusicScraper. Searched-Artist: Ed Sheeran Found: 1 matches
2012-12-23 20:22:35.893035 [Debug][MPMain(1)]: Single Artist Match Found
2012-12-23 20:22:36.591074 [Debug][MPMain(1)]: Trying to parse html for artist: Ed Sheeran
2012-12-23 20:22:37.147106 [Debug][MPMain(1)]: WindowManager: unroute to MediaPortal.Dialogs.GUIDialogProgress:101->MediaPortal.GUI.Music.GUIMusicGenres:504
2012-12-23 20:22:37.337117 [Debug][MPMain(1)]: WindowManager: route MediaPortal.GUI.Music.GUIMusicGenres:504->MediaPortal.GUI.Music.GUIMusicArtistInfo:2011
2012-12-23 20:22:37.338117 [Debug][MPMain(1)]: DialogWindow: MediaPortal.GUI.Music.GUIMusicArtistInfo init
2012-12-23 20:22:37.413121 [Debug][MPMain(1)]: Picture: Fast loaded texture C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Artists\Ed Sheeran.jpg
2012-12-23 20:22:45.916608 [Debug][MPMain(1)]: WindowManager: unroute to MediaPortal.GUI.Music.GUIMusicArtistInfo:2011->MediaPortal.GUI.Music.GUIMusicGenres:504
2012-12-23 20:22:46.243627 [Debug][MPMain(1)]: Windowmanager: Goto previous window
2012-12-23 20:22:46.625648 [Debug][MPMain(1)]: Window: MediaPortal.GUI.Music.GUIMusicGenres deinit
2012-12-23 20:22:46.632649 [Debug][MPMain(1)]: TextureManager: CleanupThumbs()
2012-12-23 20:22:46.661650 [Debug][MPMain(1)]: Window: WindowPlugins.home.GUIBasicHome init
2012-12-23 21:51:58.697786 [Debug][(10)]: fileSystemWatcher_Created file c:\programdata\team mediaportal\mediaportal\thumbs\music\albums\Tina Turner-Tina!.jpg
2012-12-23 21:51:58.728986 [Debug][(12)]: CreateThumbnail : FileExistsInCache updated with new file: C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Albums\Tina Turner-Tina!.jpg
2012-12-23 21:51:58.791387 [Debug][(10)]: fileSystemWatcher_Created file c:\programdata\team mediaportal\mediaportal\thumbs\music\albums\Tina Turner-Tina!L.jpg
2012-12-23 21:51:58.822587 [Debug][(12)]: CreateThumbnail : FileExistsInCache updated with new file: C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Albums\Tina Turner-Tina!L.jpg
2012-12-23 21:51:59.118987 [Info.][MusicScan(11)]: Musicdatabasereorg: Total Songs: 295. 0 added / 295 updated / 0 skipped
2012-12-23 21:51:59.118987 [Info.][MusicScan(11)]: Musicdatabasereorg: Processed 295 tracks in: 00:00:23 (0,08053844 seconds per track)
2012-12-23 21:51:59.118987 [Info.][MusicScan(11)]: Musicdatabasereorg: Removing non existing songs from the database
2012-12-23 21:51:59.118987 [Info.][MusicScan(11)]: Musicdatabasereorg: starting song cleanup for 295 songs
2012-12-23 21:51:59.150187 [Info.][MusicScan(11)]: Musicdatabasereorg: DeleteNonExistingSongs completed. Removed 0 non-existing songs
2012-12-23 21:51:59.150187 [Info.][MusicScan(11)]: Musicdatabasereorg: Create Artist Thumbs
2012-12-23 21:51:59.165787 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: new items found waiting for caching: 1
2012-12-23 21:51:59.165787 [Debug][FileExistsCache Thread(8)]: InsertFilesIntoCacheAsynch: pre-scanning dir : c:\programdata\team mediaportal\mediaportal\thumbs\music\artists
2012-12-23 21:51:59.165787 [Debug][FileExistsCache Thread(8)]: AddWatcher c:\programdata\team mediaportal\mediaportal\thumbs\music\artists
2012-12-23 21:51:59.165787 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: no more items to cache, suspending thread.: 1
2012-12-23 21:51:59.181387 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: new items found waiting for caching: 1
2012-12-23 21:51:59.181387 [Debug][FileExistsCache Thread(8)]: InsertFilesIntoCacheAsynch: pre-scanning dir : c:\programdata\team mediaportal\mediaportal\thumbs\music\folder
2012-12-23 21:51:59.181387 [Debug][FileExistsCache Thread(8)]: AddWatcher c:\programdata\team mediaportal\mediaportal\thumbs\music\folder
2012-12-23 21:51:59.181387 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: new items found waiting for caching: 1
2012-12-23 21:51:59.181387 [Debug][FileExistsCache Thread(8)]: InsertFilesIntoCacheAsynch: pre-scanning dir : f:\musik\die ultimative chartshow\rockballaden
2012-12-23 21:51:59.196987 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: no more items to cache, suspending thread.: 1
2012-12-23 21:51:59.196987 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.196987 [Debug][MusicScan(11)]: Utils: CreateFolderPreviewThumb for C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Artists\10 CC.jpg took 0 ms
2012-12-23 21:51:59.196987 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.196987 [Debug][MusicScan(11)]: Utils: CreateFolderPreviewThumb for C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Artists\3 Doors Down.jpg took 0 ms
2012-12-23 21:51:59.196987 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: new items found waiting for caching: 1
2012-12-23 21:51:59.196987 [Debug][FileExistsCache Thread(8)]: InsertFilesIntoCacheAsynch: pre-scanning dir : f:\musik\die ultimative chartshow\lovesongs der 80er
2012-12-23 21:51:59.212587 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.212587 [Debug][MusicScan(11)]: Utils: CreateFolderPreviewThumb for C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Artists\A-Ha.jpg took 12 ms
2012-12-23 21:51:59.212587 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: no more items to cache, suspending thread.: 1
2012-12-23 21:51:59.212587 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.446588 [Info.][MusicScan(11)]: Musicdatabasereorg: Create Genre Thumbs
2012-12-23 21:51:59.446588 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: new items found waiting for caching: 1
2012-12-23 21:51:59.446588 [Debug][FileExistsCache Thread(8)]: InsertFilesIntoCacheAsynch: pre-scanning dir : c:\programdata\team mediaportal\mediaportal\thumbs\music\genre
2012-12-23 21:51:59.446588 [Debug][FileExistsCache Thread(8)]: AddWatcher c:\programdata\team mediaportal\mediaportal\thumbs\music\genre
2012-12-23 21:51:59.446588 [Debug][FileExistsCache Thread(8)]: FileExistsCacheThread: no more items to cache, suspending thread.: 1
2012-12-23 21:51:59.462188 [Debug][MusicScan(11)]: MusicDatabase: GetSongsByFilter - SQL: SELECT * FROM tracks WHERE strGenre LIKE '%| New Age |%' GROUP BY strAlbum ORDER BY iYear DESC, Filter: genre
2012-12-23 21:51:59.462188 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.462188 [Debug][MusicScan(11)]: Utils: CreateFolderPreviewThumb for C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Genre\New Age.jpg took 0 ms
2012-12-23 21:51:59.462188 [Debug][MusicScan(11)]: MusicDatabase: GetSongsByFilter - SQL: SELECT * FROM tracks WHERE strGenre LIKE '%| Pop |%' GROUP BY strAlbum ORDER BY iYear DESC, Filter: genre
2012-12-23 21:51:59.462188 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!
2012-12-23 21:51:59.462188 [Debug][MusicScan(11)]: Utils: CreateFolderPreviewThumb for C:\ProgramData\Team MediaPortal\MediaPortal\Thumbs\Music\Genre\Pop.jpg took 0 ms
2012-12-23 21:51:59.462188 [Debug][MusicScan(11)]: MusicDatabase: GetSongsByFilter - SQL: SELECT * FROM tracks WHERE strGenre LIKE '%| Rock |%' GROUP BY strAlbum ORDER BY iYear DESC, Filter: genre
2012-12-23 21:51:59.462188 [Warn.][MusicScan(11)]: Utils: Your skin does not supply previewbackground.png to create folder preview thumbs!