WordPress ギャラリーショートコードのカスタマイズ（サムネイル＋大きい画像＋画像が表示されたらGoogleアナリティクスに送信） ref: http://qiita.com/gatespace/items/568a3c601cf762207a87

file1.html

<div id='gallery-1' class='gallery galleryid-555 gallery-columns-3 gallery-size-thumbnail'>
    <div class="gallery-bxslider">
        <figure class='slider-item' data-href='/archives/555/canola2'>
            <div class='slider-icon landscape'>
                <img width="640" height="480" src="http://example.com/wp-content/uploads/2011/01/canola2.jpg" />
            </div>
            <figcaption class='slider-caption-text slider-caption' id='gallery-1-611'>
                caption text.
            </figcaption>
        </figure>
        (.slider-item の繰り返し)
    </div>
    <div id="bx-pager-1" class="slider-pager">
        <a data-slide-index="0" href="#" class="slider-pager-icon"><img width="150" height="150" src="http://example.com/wp-content/uploads/2011/01/canola2-150x150.jpg" class="attachment-thumbnail size-thumbnail" alt="canola" aria-describedby="gallery-1-611" /></a>
        (.slider-pager-icon の繰り返し)
    </div>
</div>      

gallery-style.css

.slider-pager .slider-pager-icon {
    display: inline-block;
    width: 50px;
    height: 50px;
    border: 3px solid #ffffff;
}
.slider-pager .slider-pager-icon.active,
.slider-pager .slider-pager-icon:hover,
.slider-pager .slider-pager-icon:focus,
.slider-pager .slider-pager-icon:active {
    border: 3px solid #e60012;
}

my-gallery-shortcode.php

<?php
/**
 * Remove default gallery shortcode function.
 * Add custom gallery shortcode function.
 */
remove_shortcode( 'gallery', 'gallery_shortcode' );
add_shortcode( 'gallery', 'my_gallery_shortcode' );
function my_gallery_shortcode( $attr ) {
    $post = get_post();

    static $instance = 0;
    $instance++;

    if ( ! empty( $attr['ids'] ) ) {
        // 'ids' is explicitly ordered, unless you specify otherwise.
        if ( empty( $attr['orderby'] ) ) {
            $attr['orderby'] = 'post__in';
        }
        $attr['include'] = $attr['ids'];
    }

    /**
     * Filter the default gallery shortcode output.
     *
     * If the filtered output isn't empty, it will be used instead of generating
     * the default gallery template.
     *
     * @since 2.5.0
     * @since 4.2.0 The `$instance` parameter was added.
     *
     * @see gallery_shortcode()
     *
     * @param string $output   The gallery output. Default empty.
     * @param array  $attr     Attributes of the gallery shortcode.
     * @param int    $instance Unique numeric ID of this gallery shortcode instance.
     */
    $output = apply_filters( 'post_gallery', '', $attr, $instance );
    if ( $output != '' ) {
        return $output;
    }

    $html5 = current_theme_supports( 'html5', 'gallery' );
    $atts = shortcode_atts( array(
        'order'      => 'ASC',
        'orderby'    => 'menu_order ID',
        'id'         => $post ? $post->ID : 0,
        'itemtag'    => $html5 ? 'figure'     : 'dl',
        'icontag'    => $html5 ? 'div'        : 'dt',
        'captiontag' => $html5 ? 'figcaption' : 'dd',
        'columns'    => 3,
        'size'       => 'thumbnail',
        'include'    => '',
        'exclude'    => '',
        'link'       => ''
    ), $attr, 'gallery' );

    $id = intval( $atts['id'] );

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

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

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

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

    $itemtag = tag_escape( $atts['itemtag'] );
    $captiontag = tag_escape( $atts['captiontag'] );
    $icontag = tag_escape( $atts['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( $atts['columns'] );
    $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
    $float = is_rtl() ? 'right' : 'left';

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

    $gallery_style = '';

    /**
     * Filter whether to print default gallery styles.
     *
     * @since 3.1.0
     *
     * @param bool $print Whether to print default gallery styles.
     *                    Defaults to false if the theme supports HTML5 galleries.
     *                    Otherwise, defaults to true.
     */
    if ( apply_filters( 'use_default_gallery_style', ! $html5 ) ) {
        $gallery_style = "
        <style type='text/css'>
            #{$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>\n\t\t";
    }

    $size_class = sanitize_html_class( $atts['size'] );
    $gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";

    /**
     * Filter the default gallery shortcode CSS styles.
     *
     * @since 2.5.0
     *
     * @param string $gallery_style Default CSS styles and opening HTML div container
     *                              for the gallery shortcode output.
     */
    $output = apply_filters( 'gallery_style', $gallery_style . $gallery_div );

    // ここからが本番
    $slider = '';
    $pager  = '';
    $i = 0;

    foreach ( $attachments as $id => $attachment ) {

        $attr = ( trim( $attachment->post_excerpt ) ) ? array( 'aria-describedby' => "$selector-$id" ) : '';

        $image_output = wp_get_attachment_image( $id, 'full', false, $attr );
        $image_uri    = get_attachment_link( $id );
        $image_path   = str_replace( home_url(), '', $image_uri ); // GAに送るアタッチメントページのURL（処理済）

        $pager_output = wp_get_attachment_image( $id, 'thumbnail', false, $attr );
        $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';
        }
        $slider .= "<{$itemtag} class='slider-item' data-href='{$image_path}'>";
        $slider .= "
            <{$icontag} class='slider-icon {$orientation}'><div class='slider-icon-img'>
                $image_output
            </div></{$icontag}>";
        if ( $captiontag && trim($attachment->post_excerpt) ) {
            $slider .= "
                <{$captiontag} class='slider-caption-text slider-caption' id='$selector-$id'>
                " . wptexturize($attachment->post_excerpt) . "
                </{$captiontag}>";
        }
        $slider .= "</{$itemtag}>\n";

        $pager .= '<a data-slide-index="' . $i . '" href="#" class="slider-pager-icon">';
        $pager .= $pager_output;
        $pager .= "</a>\n";

        $i ++;
    }

    $output .= '<div class="gallery-bxslider">' . "\n";
    $output .= $slider;
    $output .= "</div>\n";

    $output .= '<div id="bx-pager-' . $instance . '" class="slider-pager">' . "\n";
    $output .= $pager;
    $output .= "</div>\n";

    $output .= "</div>\n";

    return $output;
}

slider.js

(function($){
    // BxSlider gallery
    var gallerybox = $('.gallery');
    var gallery, pagerObj, pagerID, slideurl;
    if ( gallerybox.length > 0 ) {
        gallerybox.each(function(){
            gallery        = $(this).children('.gallery-bxslider');
            pagerObj       = $(this).children('.slider-pager');
            pagerID        = pagerObj.attr('id');

            gallery.bxSlider({
                pagerCustom:  '#' + pagerID,
                onSliderLoad: function( currentIndex ) {
                    slideurl = gallery.children().eq( currentIndex + 1 ).attr( 'data-href' );
                    console.log(slideurl);
                    if ( typeof ga == 'function' ) { 
                        ga( 'send', 'pageview', slideurl );
                    }
                },
                onSlideBefore: function($slideElement, oldIndex, newIndex) {
                    slideurl = $slideElement.attr( 'data-href' );
                    console.log(slideurl);
                    if ( typeof ga == 'function' ) { 
                        ga( 'send', 'pageview', slideurl );
                    }
                }
            });
        });
    }
})(jQuery);