The captcha image is broken (no image appearing). The other components of captcha (links, error matching code etc) seem to work correctly

In Controller:

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


In View:

<?php if (CCaptcha::checkRequirements()): ?>
<div class="row">
<?php echo $form->labelEx($model, 'verifyCode'); ?>
<div>
<?php $this->widget('CCaptcha'); ?>
<?php echo $form->textField($model, 'verifyCode'); ?>
</div>
<div class="hint">Please enter the letters as they are shown in the image above.

Letters are not case-sensitive.</div>
<?php echo $form->error($model, 'verifyCode'); ?>
</div>
<?php endif; ?>


In Model:

public $verifyCode;
....
public function rules() {
return array(
....
....
array('verifyCode', 'captcha', 'allowEmpty' => !CCaptcha::checkRequirements()),
);
}


Why I have this problem?