WordPress | Portrait or Landscape

wp_image_orientation.php

<?php
$post_thumbnail_id = get_post_thumbnail_id();
$image_attributes = wp_get_attachment_image_src( $post_thumbnail_id, 'full' );
if ( $image_attributes ) {
    $width = $image_attributes[1]; 
    $height = $image_attributes[2]; 
    if ( $width > $height ) {
        echo get_the_post_thumbnail( $post_id, 'medium', array( 'class' => 'landscape' ) );
    } elseif ( $width < $height ) {
        echo get_the_post_thumbnail( $post_id, 'medium', array( 'class' => 'portrait' ) );
    } else {
        // If we are here the width and height must be equal
        echo get_the_post_thumbnail( $post_id, 'medium', array( 'class' => 'square' ) );
    }
}
?>