adding ajax load more to display images from meta box

It can be done just following regular way to use Ajax in WordPress.

https://codex.wordpress.org/AJAX_in_Plugins

First register a script file and also create an AJAX Call.

wp_enqueue_script( 'meta-ajaxscript', get_stylesheet_directory_uri() . '/js/ajax-init.js', array( 'jquery' ), '', true );
wp_localize_script( 'meta-ajaxscript', 'ajaxMeta', array(
    'ajaxurl' => admin_url( 'admin-ajax.php' )
));

Second, Wrap your metadata with a parent element by class or ID

<div id="metaWrapper">

    <?php  
    $images = rwmb_meta( 'tj_file_advanced', 'type=image&size=YOURSIZE'); 
    $ppp = 3;
    foreach (array_slice($images, 0, $ppp) as $image) { ?>
    <div class="col-md-4">
        <div class="boxe">
            <?php echo "<img src="https://wordpress.stackexchange.com/questions/201684/{$image["full_url']}'/>"; ?>
        </div>
    </div>
    <?php } ?>

</div>

<div class="loadmore"></div>

Third write down some ajax call script into ajax-init.js.

$( ".loadmore" ).on( "click", function(e) {
  e.preventDefault();

  var $container = $('#metaWrapper'),
      ppp = 3,
      number = $container.find( '> .col-md-4').length,
      page = number / ppp;

  $.ajax({
    url: ajaxMeta.ajaxurl,
    type: 'post',
    data: {
      'page': page,
      'ppp': ppp,
       action: 'meta_fetch'
    },
    success: function( response ) {
      if(response == "")
          {
            // no content was found
            $('.loadmore').hide();
          }
          else
          {
            console.log(response);
            $container.append( response );
          }
      }
  });

  return false;
});

Now set up a PHP function to handle the AJAX request.

add_action( 'wp_ajax_nopriv_meta_fetch', 'wpex_metadata_fetch' );
add_action( 'wp_ajax_meta_fetch', 'wpex_metadata_fetch' );

function wpex_metadata_fetch() {

  $images = rwmb_meta( 'tj_file_advanced', 'type=image&size=YOURSIZE');

  $page = (int) $_POST['page'];
  $ppp  = (int) $_POST['ppp'];

  foreach (array_slice($images, $page*$ppp, $ppp) as $image) { ?>
  <div class="col-md-4">
      <div class="boxe">
          <?php echo "<img src="https://wordpress.stackexchange.com/questions/201684/{$image["full_url']}'/>"; ?>
      </div>
  </div>
  <?php
  } 

  die();

}

hope it makes sense and happy Codding!