The Extra Theme limits the height of featured images on posts to 640px by default. Images taller than this will be cropped top and bottom, as shown in the before picture above.

If you’d like to increase the maximum height of the featured images on posts to, say 1000px, you can do by adding the following PHP code into Extra’s functions.php file, or into the functions.php file of a child theme (N.B. it’s advisable to take a backup of your site before editing PHP code as PHP errors can cause your site to become inaccessible):

// Prevent featured post cropping 
function disable_cropping_on_featured_images() {
    add_image_size( 'extra-image-single-post', 1280, 1000, true );
add_action('after_setup_theme', 'disable_cropping_on_featured_images', 11);

If you wish to allow even taller images, simply increase the “1000” in the above.

Important: The fix above will apply only to newly uploaded featured images. To apply it to existing featured images you must regenerate the thumbnails using a plugin such as Force Regenerate Thumbnails. Thanks to Jessica for pointing this out.