Copy of message http://forums.slimdevices.com/showthread.php?t=37104 
If the response given is right, this is a known problem but I didn't find it open here in bugzilla

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Hi,

I'm annoyed from the beginning with a little bug. From now, I was used to "clear library and rescan everything" but as my song collection is growing this is becoming more and more annoying.

Here is my problem:
- clear library and rescan everything" always works perfectly wether the artwork is a song artwork in a flac file, a folder.jpg in my directory and everything.
- when using "loook for new and changed music", song covers are displaying in the "now playing"; album covers are displayed well when I click on the album but some² thumbnails are broken displaying the grey question mark, but the img name isn't "generic":
http://<myserver>:9000/music/568/thumb_100x100_f_000000.jpg

Is this a known problem? (I've look around but only found "complete artwork broken" problems, not partial like this one.
Is there a workaround?
Should I move this problem right now in bugzilla?

²: around 10 broken covers on 150 albums
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I don't use itunes plugins and most of my music collection is flac with folder.jpg + cover art in flac files. A little part of my collection is MP3 with folder.jpg and cover art in mp3 files.