Crop of featured image

This is my website, the theme i’m using is hatch

http://fbzphotography.altervista.org/category/beauty/

You can see what my problem is: I can’t crop the image used as a thumbnail. I go in the editor and select “thumbnail only”, in the preview on the right the image appears to be cropped, but as I go on my homepage nothing has changed.
Do someone know why?