Currently mediawiki constrains image size primarily by width, which doesn't work so well for images which are taller than they are wide. There is no way to ask for an image which has a height equal to the "default thumbnail size" (without explicitly specifying a size in px)!
It would be nice to fix this. VisualEditor/Parsoid would really prefer to use a simplified model where all bounding boxes are square, including the default thumbnail size.
Please see https://www.mediawiki.org/wiki/Requests_for_comment/Square_bounding_boxesfor two concrete proposals for how this might be achieved, and discuss!
Thanks... --scott