It looks like my TB installation can't show any subcategory picture.
This is a subcategory (Main category > subcategory).
It keeps showing this blank image...the problem is that TB looks for a picture that doesn't exist : /shop/img/c/6-0_thumb.jpg
I tried creating a different subcategory (nested inside another main category), and it keeps using the same path as before : /shop/img/c/6-0_thumb.jpg
This means that I can't manually create a thumbnail and put it into that directory, because my img would be used for two different subcategories!!