Nesting limit... thumbnail is always the first picture. So post a picture with some clothes on, or something as the first one... and the thumbnail will be safe.
EDIT: I've only had it choose the third picture if I posted it and that third picture was the 1st. Then I edited it and added more photos before that. It stuck with the original one. That doesn't mean it can't happen. I just haven't seen it happen. Yes and I agree on the BETA thing. It will be addressed.
It's not always the first picture. https://steemit.com/introduceyourself/@fubar-bdhr/diaspora-shattered-armistice-battlestar-galactica-space-combat-simulator its the third pic. I had to edit it several times to try to get the right pic to show and move things around.