If WD would hurry up and upgrade the Twonky version this issue would be solved. I have loaded Twonky V6 on a PC and when the WDTV connects via media server the thumbnails work perfectly.
WD has been asked to fix this issue for a number of years and refuses to do so, even though to upgrade Twonky from 5 to 6 is free.
The other NAS suppliers such as QNAP did this a couple of years ago (see link)
WD get with the times and give us current updates or we will go else where !!