i have unable show hidden categories/images but doesnt work again. the thumbnails that i want is hidden...
I'm not sure if I understand you correctly.
So once again:
If you use hidden images for your manual category thumbnails you have to enable the module option 'Show hidden categories/images'.
Furthermore these hidden images have to be published
(please check that in image manager, it seems to me that this is not the case).
The 'Thumbnail display mode' 'Manual setting' should work than!