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" />