That’s very strange. On my other blog, they’re perferctly square:
https://www.wincert.net/cast/kb4549951-causes-more-issues-for-windows-10-users/
I see it’s different on the theme demo.
It would make a lot of sense to have the option of using the same ratio both as thumbnail and in the related posts section to avoid the cropped text problem (and it must be somehow possible; see wincert blog).
