Convert Gallery into Cycle Slideshow.

gallery-slideshow.php

<?php
/* Gallery Shortcode */
function gallery_to_slideshow($atts) {
    global $post;
    $slider_html = '';
    $ids = NULL;
    $image_size = NULL;
    
    extract( shortcode_atts( array( 'ids' => '', 'image_size' => 'thumbnail' ), $atts) );
    $attachment_ids = explode(',', $ids);
    
    if ( isset( $attachment_ids ) && !empty( $attachment_ids ) ) {
        $slider_html .= '<div class="rtp-slideshow-container">';
        //$slider_html .='<a href="#" class="rtp-close">'. __('Close', 'rtPanel') .'</a>';
        $slider_html .= '<ul class="cycle-slideshow rtp-main-slideshow"
                                    data-cycle-slides="> li"
                                    data-cycle-timeout="4000"
                                    data-cycle-log="false"
                                    data-cycle-prev=".cycle-prev"
                                    data-cycle-next=".cycle-next"
                                    data-cycle-fx="fade"> ';

                        foreach ( $attachment_ids as $attachment_id ) {
                            $attachment = get_post($attachment_id);
                            $slide = wp_get_attachment_image_src($attachment_id, $image_size);

                            $slider_html .= '<li class="slide">';
                                $slider_html .='<img src="' . $slide[0] .'" alt="' . $attachment->post_title . '" />';
                                $slider_html .= '<h2 class="attachment-title">' . $attachment->post_title . '</h2>';
                                $slider_html.='<div class="image-description">' . $attachment->post_content. '</div>';
                            $slider_html.='</li>';
                        }
        
        $slider_html .= '</ul>';
        $slider_html .= '<div class="rtp-prev cycle-prev">'. __('&lt;&lt;Prev', 'rtPanel') .'</div>';
        $slider_html .= '<div class="rtp-next cycle-next">'. __('Next&gt;&gt;', 'rtPanel') .'</div>';
        $slider_html .= '</div>';//End of Div rtp-slideshow-container
        
        return $slider_html;
    }
}

/* Override Gallery */
remove_shortcode('gallery');
add_shortcode('gallery', 'gallery_to_slideshow');

Note: You must use cycle2 jQuery plugin for slideshow. (http://www.malsup.com/jquery/cycle2/download/)