caption-class.php

<?php
/*
Plugin Name: Add credit-class to image captions
Description: By adding ":credits-caption:" into your image caption, the class "credits-caption" will get added to the div surrounding the image.
Author: Jan Dembowski
Author URI: http://blog.dembowski.net/
*/

add_filter( 'img_caption_shortcode', 'mh_caption_class_hack', 10, 3 );
function mh_caption_class_hack( $output, $attr, $content ) {

        if ( $output != '' )
                return $output;

        extract(shortcode_atts(array(
                'id'    => '',
                'align' => 'alignnone',
                'width' => '',
                'caption' => ''
        ), $attr));

        if ( 1 > (int) $width || empty($caption) )
                return $content;

        if ( $id ) $id = 'id="' . esc_attr($id) . '" ';

        if ( !preg_match( '/:credits-caption:/' , $caption . '/' , $mh_dont_care ) ) {
                return '<div ' . $id . 'class="wp-caption ' . esc_attr($align) . '" style="width: ' . (10 + (int) $width) . 'px">'
                . do_shortcode( $content ) . '<p class="wp-caption-text">' . $caption . '</p></div>';
        } else {
                $caption = str_replace( ':credits-caption:', '' , $caption );
                return '<div ' . $id . 'class="wp-caption credits-caption ' . esc_attr($align) . '" style="width: ' . (10 + (int) $width) . 'px">'
                . do_shortcode( $content ) . '<p class="wp-caption-text">' . $caption . '</p></div>';
        }
}