How to set featured image post thumbnail to keep image proportion

Hi,

I’m just wondering if anyone knows how to do this. I have looked at various ways. What I am looking for is a fix that shows a post thumbnail of the featured image in it’s full proportion (i.e. scale to fit and not crop and centre).

I am testing a theme called Rehub and when I list my posts on a category page, the featured image becomes scaled up and cropped to centre.

https://www.whenisthenext.co.uk/category/affiliate-test/

Unfortunately, the API i am using to pull these images only supplies me with a thumbnail image (of varying sizes), so they are not large. This example shows one which is typically 300px x 92px, so when it enlarges in the thumbnail view, it pixelates.

The class appears to be: class=”lazyimages”

Can anyone help? Many thanks!