[Theme: Hueman] Improving post thumbnail quality?

Hi. Our current site uses Hueman, and it remains wonderful for our purposes. However, I’ve noticed that the featured images for our articles are very compressed on the index pages. Take for example this page that shows one of the clearly problematic images: https://www.laajakuva.com/?s=dreamcatcher

The poster for Dreamcatcher is very high quality, with both width and height reaching well over 1000 pixels in the source image used for the featured image. The thumbnail however, as you can see, is the furthest thing from the crisp and clean original. Is there a resizing or compression setting somewhere, and how do I turn it off?