gistfile1.php

function maor_image_proportions_classes($attr, $attachment) {
	$image = wp_get_attachment_image_src( $attachment->ID, 'full', false );

	if ( ! $image )
		return $attr;

	list( $src, $width, $height ) = $image;

	$attr['class'] .= ( $height > $width ) ? ' wp-image-portrait' : ' wp-image-landscape';

	return $attr;
}
add_filter( 'wp_get_attachment_image_attributes', 'maor_image_proportions_classes', 10, 2 );