I am not sure if this was mentioned before, but since the first npvr version, there is a bug/problem with displaying thumbnails.
As you can see above here, there are a few empty fields. Normally there are not so much of them (in most cases 2 or 3... sometimes it will be displayed a folder icon instead the right picture -> cache problem).
Now you can see, that the picture will be displayed, if i highlight the file.
And here, this one is new since nPVR 2.03, you can see that the picture disappears after moving the selection to another video (before 2.03 the picture stayed on the display if it was onetime highlighted).
Of course i deleted the thumbnail cache a few times, but this didn't helped really. Is there a file limitation of thumbnails?
Another nice feature (more a wish) were the ability to use .png files as folder graphics (because of their transparency channel).