My header.php code

<?php
    // Check to see if the header image has been removed
    $header_image = get_header_image();
    if ( ! empty( $header_image ) ) :
        if ( function_exists( 'get_custom_header' ) ) {
            // We need to figure out what the minimum width should be for our featured image.
            // This result would be the suggested width if the theme were to implement flexible widths.
            $header_image_width = get_theme_support( 'custom-header', 'width' );
        }
        ?>
<a href="<?php echo esc_url( home_url( '/' ) ); ?>">
    <?php
        // The header image
        // Check if this is a post or page, if it has a thumbnail, and if it's a big one
        if ( is_singular() && has_post_thumbnail( $post->ID ) &&
                ( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), array( $header_image_width, $header_image_width ) ) ) &&
                $image[1] >= $header_image_width ) :
            // Houston, we have a new header image!
            echo get_the_post_thumbnail( $post->ID, 'post-thumbnail' );
        else :
            if ( function_exists( 'get_custom_header' ) ) {
                $header_image_width  = get_custom_header()->width;
                $header_image_height = get_custom_header()->height;
            }
            ?>
        <img src="<?php echo esc_url( $header_image ); ?>" class="header-image" width="<?php echo get_custom_header()->width; ?>" height="<?php echo get_custom_header()->height; ?>" alt="" />
    <?php endif; // end check for featured image or standard header ?>
</a>
<?php endif; // end check for removed header image ?>

Output of this code:

<img width="624" height="187" src="http://matulaitismission.org/wp-content/uploads/2013/01/Matulaicio-Misijos-Prakartele-624x187.jpg" class="attachment-post-thumbnail wp-post-image" alt="Matulaicio-Misijos-Prakartele" />