ギャラリーの写真の上に文字かバナーを入れたいです

1）テーマの functions.php に以下のようなコードを書けばOK。
※デフォルトのタグが dl, dt, dd なのでアイコン用タグ（dt）の中にタイトルを入れていますが、お好みで変更してください。

apply_filters( 'post_gallery', 'my_post_gallery', 10, 2 );
function my_post_gallery( $output, $attr ) {
  // We're trusting author input, so let's at least make sure it looks like a valid orderby statement
  if ( isset( $attr['orderby'] ) ) {
    $attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
    if ( !$attr['orderby'] )
      unset( $attr['orderby'] );
  }

  extract(shortcode_atts(array(
    'order'      => 'ASC',
    'orderby'    => 'menu_order ID',
    'id'         => $post ? $post->ID : 0,
    'itemtag'    => 'dl',
    'titletag'   => 'div', // ★追加
    'icontag'    => 'dt',
    'captiontag' => 'dd',
    'columns'    => 3,
    'size'       => 'thumbnail',
    'include'    => '',
    'exclude'    => '',
    'link'       => ''
  ), $attr, 'gallery'));

  $id = intval($id);
  if ( 'RAND' == $order )
    $orderby = 'none';

  if ( !empty($include) ) {
    $_attachments = get_posts( array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );

    $attachments = array();
    foreach ( $_attachments as $key => $val ) {
      $attachments[$val->ID] = $_attachments[$key];
    }
  } elseif ( !empty($exclude) ) {
    $attachments = get_children( array('post_parent' => $id, 'exclude' => $exclude, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
  } else {
    $attachments = get_children( array('post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
  }

  if ( empty($attachments) )
    return '';

  if ( is_feed() ) {
    $output = "\n";
    foreach ( $attachments as $att_id => $attachment )
      $output .= wp_get_attachment_link($att_id, $size, true) . "\n";
    return $output;
  }

  $itemtag = tag_escape($itemtag);
  $captiontag = tag_escape($captiontag);
  $icontag = tag_escape($icontag);
  $valid_tags = wp_kses_allowed_html( 'post' );
  if ( ! isset( $valid_tags[ $itemtag ] ) )
    $itemtag = 'dl';
  if ( ! isset( $valid_tags[ $captiontag ] ) )
    $captiontag = 'dd';
  if ( ! isset( $valid_tags[ $icontag ] ) )
    $icontag = 'dt';

  $columns = intval($columns);
  $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
  $float = is_rtl() ? 'right' : 'left';

  $selector = "gallery-{$instance}";

  $gallery_style = $gallery_div = '';
  if ( apply_filters( 'use_default_gallery_style', true ) )
    $gallery_style = "
    <style type='text/css'>
      #{$selector} {
        margin: auto;
      }
      #{$selector} .gallery-item {
        float: {$float};
        margin-top: 10px;
        text-align: center;
        width: {$itemwidth}%;
      }
      #{$selector} img {
        border: 2px solid #cfcfcf;
      }
      #{$selector} .gallery-caption {
        margin-left: 0;
      }
      /* see gallery_shortcode() in wp-includes/media.php */
    </style>";
  $size_class = sanitize_html_class( $size );
  $gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
  $output = apply_filters( 'gallery_style', $gallery_style . "\n\t\t" . $gallery_div );

  $i = 0;
  foreach ( $attachments as $id => $attachment ) {
    if ( ! empty( $link ) && 'file' === $link )
      $image_output = wp_get_attachment_link( $id, $size, false, false );
    elseif ( ! empty( $link ) && 'none' === $link )
      $image_output = wp_get_attachment_image( $id, $size, false );
    else
      $image_output = wp_get_attachment_link( $id, $size, true, false );

    $image_meta  = wp_get_attachment_metadata( $id );

    $orientation = '';
    if ( isset( $image_meta['height'], $image_meta['width'] ) )
      $orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';

    $title = wptexturize($attachment->post_title); // ★タイトル（またはバナー）を代入

    $output .= "<{$itemtag} class='gallery-item'>";
    // ★3行下でアイコン用タグの中にタイトルを入れています！
    $output .= "
      <{$icontag} class='gallery-icon {$orientation}'>
      <{$titletag} class='gallery-title'>$title</{$titletag}>
        $image_output
      </{$icontag}>";
    if ( $captiontag && trim($attachment->post_excerpt) ) {
      $output .= "
        <{$captiontag} class='wp-caption-text gallery-caption'>
        " . wptexturize($attachment->post_excerpt) . "
        </{$captiontag}>";
    }
    $output .= "</{$itemtag}>";
    if ( $columns > 0 && ++$i % $columns == 0 )
      $output .= '<br style="clear: both" />';
  }

  $output .= "
      <br style='clear: both;' />
    </div>\n";

  return $output;
}