验证码不能以yii显示图像

Question

I try CAPTCHA code in my view page ,

Model Rule

public function rules()
    {
        return array(
             array('verifyCode', 'captcha', 'allowEmpty'=>extension_loaded('gd')),
        );
    }

below code about view page control..

<?php if(extension_loaded('gd')): ?>
            <div class="row">
                <?php echo $form->labelEx($model,'verifyCode'); ?>
                <?php echo $form->textField($model,'verifyCode'); ?>
                <div>
                    <?php $this->widget('CCaptcha'); ?>
                </div>
                <div class="hint">Please enter the letters as they are shown in the image above.
                <br/>Letters are not case-sensitive.</div>
            </div>
        <?php endif; ?>

Controller action

public function actions()
    {
        return array(
            'captcha'=>array(
                'class'=>'CCaptchaAction',
                'backColor'=>0xFFFFFF,
            ),

            'page'=>array(
                'class'=>'CViewAction',
            ),
        );
    }

And Controller Access rule..

public function accessRules()
        {
            return array(
                array('allow', 
                    'actions'=>array('index','view','create','captcha'),
                    'users'=>array('*'),
                    )
                );
        } 

But not display any Imgae , help will appriciate.