I’m lazy and I like to try and figure out how to get the results I want without a lot of work. In the recent past I’ve uploaded featured images that were 1200 pixels wide and varying heights. I’m playing with a child theme of Twenty Sixteen and wanted to center crop the existing featured images.
I could have re-sampled the cropped images one at a time in Photoshop Elements or ImageMagick (for some CLI bash shell fun). Or I could have used the Regenerate Thumbnails plugin and let that go. Instead I messed with CSS until it worked the way I liked.