Add Full Width Image Support in a WordPress Theme - PHP

full-width-image-theme-support.php

<?php
/**
 * Add Theme Support for wide and full-width images.
 *
 * Add this to your theme's functions.php, or wherever else 
 * you are adding your add_theme_support() functions.
 * 
 * @action after_setup_theme
 */
function jr3_theme_setup() {
     add_theme_support( 'align-wide' );
}
add_action( 'after_setup_theme', 'jr3_theme_setup' );