Hi SteveFromAus
If you have already created child theme then you need to copy and paste header.php file from main theme’s folder to child theme’s folder.
Now find and replace below code in that header.php file in child theme folder.
<?php if ( get_header_image() ) : ?>
<?php
/**
* Filter the default twentysixteen custom header sizes attribute.
*
* @since Twenty Sixteen 1.0
*
* @param string $custom_header_sizes sizes attribute
* for Custom Header. Default '(max-width: 709px) 85vw,
* (max-width: 909px) 81vw, (max-width: 1362px) 88vw, 1200px'.
*/
$custom_header_sizes = apply_filters( 'twentysixteen_custom_header_sizes', '(max-width: 709px) 85vw, (max-width: 909px) 81vw, (max-width: 1362px) 88vw, 1200px' );
?>
<?php
if(is_front_page()){?>
<div class="header-image">
<a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home">
<img src="<?php header_image(); ?>" srcset="<?php echo esc_attr( wp_get_attachment_image_srcset( get_custom_header()->attachment_id ) ); ?>" sizes="<?php echo esc_attr( $custom_header_sizes ); ?>" width="<?php echo esc_attr( get_custom_header()->width ); ?>" height="<?php echo esc_attr( get_custom_header()->height ); ?>" alt="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>">
</a>
</div>
<?php
}
?>
<?php endif; // End header image check. ?>
Hope this will help to resolve your issue.
Let us know if any confusion.
Best Regards!!!