Western Digital TV HD Thumbnails

Western Digital TV HD 1080P Media Player

I am putting all my films on an external hard drive and I am having a few problems. Please help
I am having trouble with SOME of my thumbnails. Some work and some don’t. Especially the ones in subfolders
Can I create a folder with a thumbnail for each category e.g. Kids, TV series, films etc and have a thumbnail for each folder. As this is not currently working. Each just shows the folder on the media player and not the thumbnail
Also some of the thumbs don’t work when they are in subfolders
E.g. I have a folder called series, then a folder called tv then a folder say series1 and one for series 2 etc and then episodes in there. I want a thumbnails foe every folder.

I hope you can help me with a few of my queries!!