source: twentynineteen/2.1/image.php

<?php
/**
 * The template for displaying image attachments
 *
 * @package WordPress
 * @subpackage Twenty_Nineteen
 * @since Twenty Nineteen 1.0
 */

get_header();
?>

        <div id="primary" class="content-area">
                <main id="main" class="site-main">

                        <?php
                        // Start the Loop.
                        while ( have_posts() ) :
                                the_post();
                                ?>

                                <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

                                        <header class="entry-header">
                                        <?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
                                        </header><!-- .entry-header -->

                                        <div class="entry-content">

                                                <figure class="entry-attachment wp-block-image">
                                                <?php
                                                        /**
                                                         * Filters the default twentynineteen image attachment size.
                                                         *
                                                         * @since Twenty Sixteen 1.0
                                                         *
                                                         * @param string $image_size Image size. Default 'large'.
                                                         */
                                                        $image_size = apply_filters( 'twentynineteen_attachment_size', 'full' );

                                                        echo wp_get_attachment_image( get_the_ID(), $image_size );
                                                ?>

                                                        <figcaption class="wp-caption-text"><?php the_excerpt(); ?></figcaption>

                                                </figure><!-- .entry-attachment -->

                                                <?php
                                                the_content();
                                                wp_link_pages(
                                                        array(
                                                                'before'      => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentynineteen' ) . '</span>',
                                                                'after'       => '</div>',
                                                                'link_before' => '<span>',
                                                                'link_after'  => '</span>',
                                                                'pagelink'    => '<span class="screen-reader-text">' . __( 'Page', 'twentynineteen' ) . ' </span>%',
                                                                'separator'   => '<span class="screen-reader-text">, </span>',
                                                        )
                                                );
                                                ?>
                                        </div><!-- .entry-content -->

                                        <footer class="entry-footer">
                                        <?php
                                                // Retrieve attachment metadata.
                                                $metadata = wp_get_attachment_metadata();
                                        if ( $metadata ) {
                                                printf(
                                                        '<span class="full-size-link"><span class="screen-reader-text">%1$s</span><a href="%2$s">%3$s &times; %4$s</a></span>',
                                                        _x( 'Full size', 'Used before full size attachment link.', 'twentynineteen' ),
                                                        esc_url( wp_get_attachment_url() ),
                                                        absint( $metadata['width'] ),
                                                        absint( $metadata['height'] )
                                                );
                                        }
                                        ?>

                                                <?php twentynineteen_entry_footer(); ?>

                                        </footer><!-- .entry-footer -->
                                </article><!-- #post-<?php the_ID(); ?> -->

                                <?php
                                // Parent post navigation.
                                the_post_navigation(
                                        array(
                                                'prev_text' => _x( '<span class="meta-nav">Published in</span><br><span class="post-title">%title</span>', 'Parent post link', 'twentynineteen' ),
                                        )
                                );

                                // If comments are open or we have at least one comment, load up the comment template.
                                if ( comments_open() || get_comments_number() ) {
                                        comments_template();
                                }

                                // End the loop.
                                endwhile;
                        ?>

                </main><!-- .site-main -->
        </div><!-- .content-area -->

<?php
get_footer();