Example Custom Redactor Button in a Craft Plugin

gistfile1.php

<?php
namespace Craft;

class RedactorStylesPlugin extends BasePlugin
{
    public function getName()
    {
        return Craft::t('Redactor Styles');
    }

    public function getVersion()
    {
        return '0.5.0';
    }

    public function getDeveloper()
    {
        return 'Barrel Strength Design';
    }

    public function getDeveloperUrl()
    {
        return 'http://barrelstrengthdesign.com';
    }

    public function init()
    {
        // Modify CSS and JS in the Control Panel
        if (craft()->request->isCpRequest())
        {
            $css = "
            body .redactor_box .redactor_toolbar li a.redactor_btn_subscribe_link {
                background: url(data:image/png;base64,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) 50% 50% no-repeat;
            }";
            craft()->templates->includeCss($css);
        }
    }
}