WordPress/wp-content/themes/twentyfifteen/image.php

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

get_header(); ?>

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

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

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

				<nav id="image-navigation" class="navigation image-navigation">
					<div class="nav-links">
						<div class="nav-previous"><?php previous_image_link( false, __( 'Previous Image', 'twentyfifteen' ) ); ?></div><div class="nav-next"><?php next_image_link( false, __( 'Next Image', 'twentyfifteen' ) ); ?></div>
					</div><!-- .nav-links -->
				</nav><!-- .image-navigation -->

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

				<div class="entry-content">

					<div class="entry-attachment">
						<?php
							/**
							 * Filters the default Twenty Fifteen image attachment size.
							 *
							 * @since Twenty Fifteen 1.0
							 *
							 * @param string $image_size Image size. Default 'large'.
							 */
							$image_size = apply_filters( 'twentyfifteen_attachment_size', 'large' );

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

						<?php if ( has_excerpt() ) : ?>
								<div class="entry-caption">
									<?php the_excerpt(); ?>
								</div><!-- .entry-caption -->
							<?php endif; ?>

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

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

					<footer class="entry-footer">
					<?php twentyfifteen_entry_meta(); ?>
						<?php edit_post_link( __( 'Edit', 'twentyfifteen' ), '<span class="edit-link">', '</span>' ); ?>
					</footer><!-- .entry-footer -->

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

				<?php
				// If comments are open or we have at least one comment, load up the comment template.
				if ( comments_open() || get_comments_number() ) :
					comments_template();
					endif;

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

				// End the loop.
				endwhile;
			?>

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

<?php get_footer(); ?>