Thanks for the link. Looking at the site, the problem is that the images are portrait. So we are constraining a 3072x4608 image into a 200x200 square, which makes it 133x200. Then we’re ‘filling’ a 222x150 box with it. So we’re displaying an image 133 pixels wide into something 222 pixels wide.
One solution here is to stop ‘filling’ the banner with the image, and ‘fit’ it instead. That’s the approach we take in the topic list thumbnails:
Adding a new setting to the theme component for this would be pr-welcome if someone would like to work on it.