This is how the CSS work to make the image round.
https://www.screencast.com/t/7oXhDxGzZt
https://www.screencast.com/t/txYQsqmfY
As you can see, the CSS shapes the vertical image to square first by limiting its size to 160px by 160px, then add border radius to make it round.
But the CSS doesn’t work for the landscape image, when the width reaches 160px, the height is not enough to reach 160px if the ratio remains.
That’s why square image is preferred, you can crop it yourself instead of letting CSS crop for you.