The Extra Theme limits the height of post images on the homepage (and elsewhere that post listings are displayed using the posts module) to 376px by default. Images taller than this will be cropped top and bottom, as shown in the before picture above.

If you’d like to prevent the images being cropped, you can increase the maximum height of the images to, say 1000px, 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 post listing image cropping 
// Change post module image max height
function disable_cropping_on_medium_images() {
    add_image_size('extra-image-medium', 627, 1000, true);
add_action('after_setup_theme', 'disable_cropping_on_medium_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 images. To apply it to existing images you must regenerate the thumbnails using a plugin such as Force Regenerate Thumbnails. Thanks to Jessica for pointing this out.