Thumbnail doesn't appear on my facebook group while it does appear on my wall

Hi, I’ve tried to make this thumbnail thing work in so many different ways, but I have no clue so far..Please help me out.
Well, I set everything right and works fine.
I mean posts are automatically shared both on my personal wall and my group.
But, the thing is the tummbnail only appears on my personal wall. I attach the screenshot using my dropbox.
https://dl.dropboxusercontent.com/u/15313086/11.png
Well, I set the featured image of course…and the image on my personal wall is the featured image.
This is the setting of WPBOOK lite.
https://dl.dropboxusercontent.com/u/15313086/22.png
Please, let me know if I set anything wrong.
Thanks.

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