First of all, you should upload high resolution images to your website. The width of the featured image should be bigger than 1024px and the height of the image should be bigger than 428px. WordPress can not crop the images properly smaller than this size.
Default featured image height is 285px (width is 1024px);
and default thumbnail height is 428px (width is 964px);
If you want to change these height values;
1) Go to Appearance->Editor
2) Open “includes/functions.php” file
4) Find “285” (or 428) and change it with any other number as you want
5) Click Update file button
After this change, you may need to upload your images again.
If you don’t want to upload the same images again, you can use the following plugin to regenerate thumbnails;