The thumbnail of a render is supposed to be representative of the render. I usually, simply reduce the render down in size to fit the thumbnail size.  Anything else would NOT be reprersentative of the render, but a chopped out block that shows nothing- which is what I had to send them this morning, because there was no way to create another thumbnail that would be representative of the render.
  In my view there was nothing wrong wirth the original thumbnail- and I have not yet received a satifactory explanation of what they beleived was wrong. Yes, the figures did not have clothes on- but the figures as rendered were not human.
 I am still waiting for a satisfactory explantion and for my render to be re-instated in my gallery.