Hello,

Thumbnail sizes for the images except the product images are controlled from Setting >> Media. As I mentioned in my previous reply.

It’s set to 150px default and that’s why the images look like that. You can try increasing the size according to your needs.

Also after changing the Image Size as per your requirements from Setting >> Media run this plugin to resize all your existing images to the new size: https://wordpress.org/plugins/regenerate-thumbnails/

Best Regards.