Thumbnail featured images on front page

The thumbnail images of the featured image on the front page are small squares. All my images are longer vertical images (Pinterest size). It seems like recently those thumbnails are now showing my whole vertical image, but the older posts still have cut off square images. Is there any way to fix the old thumbnails to make it look more uniform? To see what I am talking about – go to https://www.anxioustoddlers.com

Thanks!