Add Support for Featured Images in you WordPress theme. Requires CSS like ".vertical-image { float:left; }" to work correctly.

vertical-featured-image.php

<?php
/**
 * Add support for Vertical Featured Images
 */
if ( ! function_exists( 'mytheme_vertical_check' ) ) :
	function mytheme_vertical_check( $html, $post_id, $post_thumbnail_id, $size, $attr ) {

		$image_data = wp_get_attachment_image_src( $post_thumbnail_id , 'large' );

		//Get the image width and height from the data provided by wp_get_attachment_image_src()
		$width  = $image_data[1];
		$height = $image_data[2];

		if ( $height > $width ) {
			$html = str_replace( 'attachment-', 'vertical-image attachment-', $html );
		}
		return $html;
	}
endif;

add_filter( 'post_thumbnail_html', 'mytheme_vertical_check', 10, 5 );