如何修改验证码图像源在yii

我的captcha图像没有显示，这是它的url:

mysite.com/index.php/site/captcha?v=53b56139cb358，当我试图打开它时，我得到一个

Cannot modify header information - headers already sent by (output started at /home/patrahr/public_html/index.php:1)

函数操作:

public function actions()
    {
        return array(
            // captcha action renders the CAPTCHA image displayed on the contact page
            'captcha'=>array(
                'class'=>'CCaptchaAction',
                'backColor'=>0xFFFFFF,
            ),
            // page action renders "static" pages stored under 'protected/views/site/pages'
            // They can be accessed via: index.php?r=site/page&view=FileName
            'page'=>array(
                'class'=>'CViewAction',
            ),
        );
    }

如何修复这个错误呢?