Fourteen: Full width custom page template

Need: keep header and footer just as in Twenty-Fourteen but make the body (in the middle) full width, no sidebar, no widget, no unused white space.
Makphisto paid a developer to come up with a solution in the form of a custom page template, in order to insert a table in the middle (see topic: “Twenty Fourteen – Need Help With a Custom Page Template”).
I tried the proposed code. My need is to put a picture, not a table, in the middle. I inserted a .png picture of 1080 x 1440 px in wp-content/uploads/2014/01 and used it on this newly created custom page. The resulting page is full width but the image got redimensioned, without my will, to 1080 x 572px… even though it is still 1080 x 1440 in the /2014/01 folder; not very good looking to say the least.
I following Makphisto instructions’ but did put the template file in a child theme:
Copy and past that into Notepad, save the file as no-sidebar-full-width.php (the PHP is important!). Using an FTP program, FTP that file to your server (/www/wp-content/themes/child-2014/). Then, in WordPress admin, change the page template of your page to “No Sidebar, Full Width”.

<?php
/**
 * Template Name: No Sidebar, Full Width
 *
 */

get_header(); ?>

<style>

#secondary {
    display: none !important;
}

.site-content .entry-header, .site-content .entry-content, .site-content .entry-summary, .site-content .entry-meta, .page-content, .hentry, .entry-content {
    max-width: 1260px !important;
}

#tablepress-1 {
    width: 100% !important;
}

.site-content, .site-main .widecolumn {
margin-left:0 !important;
}

.site-content {
margin-right:0 !important;
}

.site:before {
width:0px !important;
}
</style>

	<div id="primary" class="content-area">
		<div id="content" class="site-content" role="main">
			<?php
				// Start the Loop.
				while ( have_posts() ) : the_post();
					// Include the page content template.
					get_template_part( 'content', 'page' );
					// If comments are open or we have at least one comment, load up the comment template.
					if ( comments_open() || get_comments_number() ) {
						comments_template();
					}
				endwhile;
			?>
		</div><!-- #content -->
	</div><!-- #primary -->

<?php
get_sidebar();
get_footer();

I also tried to change max-width: 1260px !important; to something different than 1260px. Doing so, I lost the ability of the main menu to stick to the top as a user scrolls down.
Help anyone for the picture to show full height?