Medium-like fluid lightbox in WordPress using Fluidbox. http://sridharkatakam.com/medium-like-fluid-lightbox-wordpress-using-fluidbox/

fluidbox-init.js

jQuery(function( $ ){
	$('a.fluidbox').fluidbox();
});

functions.php

<?php
//* Do NOT include the opening php tag

//* Enqueue Fluidbox
add_action( 'wp_enqueue_scripts', 'enqueue_fluidbox' );
function enqueue_fluidbox() {

	wp_enqueue_style( 'fluidbox-css', get_bloginfo( 'stylesheet_directory' ) . '/css/fluidbox.css', array(), CHILD_THEME_VERSION );
	wp_enqueue_script( 'fluidbox', get_bloginfo( 'stylesheet_directory' ) . '/js/jquery.fluidbox.min.js', array( 'jquery' ), '1.0.0' );
	wp_enqueue_script( 'fluidbox-init',  get_stylesheet_directory_uri() . '/js/fluidbox-init.js', array( 'fluidbox' ), '1.0.0', true );

}

sample.html

<a href="http://genesis.dev/wp-content/uploads/2014/04/88H.jpg" class="fluidbox"><img src="http://genesis.dev/wp-content/uploads/2014/04/88H-300x107.jpg" alt="88H" width="300" height="107" class="aligncenter size-medium wp-image-1540" /></a>