Up until recently, my featured thumbnails worked fine, but yesterday I published an article and the crop looked blurry. A GTmetrix confirmed the image was being resized through CSS. I tried adding a custom size (350×250) and selecting it in the Customizer, removed the dimensions and it didn’t work – the image ended up a miniature of the full size image.
a) Set the Media Attachment Size to something larger than 350px wide (but not too large), then use the width/height fields to resize down. Right now it looks like you’re using the thumbnail (150×150) and resizing up which is resulting in quality loss.
Tried method a) with several combinations, all look bad except medium_large size and 300×200, shows OK but is not cropped, is resized with CSS; that is how it is working right now. Method b) doesn’t work, it shows a thumbnail of the full image.
What I don’t understand is why the images before worked fine and the last one doesn’t.