compressed images in preview after update

my website is: https://www.fotokultur.ch

(i use german wordpress, just in case i do not use the wording as in the english version)

After the recent updates i notices that rectangular images (both Landscape and Portrait format) are compressed to fit the preview on the home page. (Images i saved or are square format where ok)

how can i prevent this (without making for each pictureposts a square preview image)?

Thanks for you help.