It is not possible to access the ITunes image through the Podcast/ITunes extension (Zend/Feed/Reader/Extension/Podcast/Entry.php).

    /**
     * Get the entry image
     *
     * @return string
     */
    public function getImage()
    {
        if (isset($this->_data['image'])) {
            return $this->_data['image'];
        }

        $image = $this->_xpath->evaluate('string(' . $this->getXpathPrefix() . '/itunes:image/@href)');

        if (!$image) {
            $image = null;
        }

        $this->_data['image'] = $image;

        return $this->_data['image'];
    }

That would be a really nice enhancement!