How can I custom and add image into feed template?

[fastcom]

Hello,

In RSS Feed managers, I add a new feed but in template box just have something: title, description, link, id, date,...

example:

{feed:date}
{feed:description}
More... ({feed:link})


Now, I need add image into feed, example:

{feed:img}


My feed create on Joomla!1.5 Platform like this

<item>
<title>Etiam a viverra tellus sed at bibendum nibh</title>
<link>http://mywebsite.com/link-to-acticle.html</link>
<guid>http://mywebsite.com/link-to-acticle.html</guid>
<description><![CDATA[<p><em><img title="Title of image" src="http://mywebsite.com/link-to-image.jpg" alt="Title of image" width="130" height="55" />Phasellus quis augue odio. Vestibulum adipiscing nunc ut eros rutrum sed aliquet massa mattis. Pellentesque eleifend, orci nec malesuada cursus, orci tellus semper justo, ut congue felis sem ut orci. Pellentesque metus quam, dapibus quis tincidunt ut, mattis ut ligula. Morbi in risus dui, sed pharetra tellus. Etiam a viverra tellus. Sed at bibendum nibh. Nulla iaculis suscipit eleifend.</em></p>
]]></description>
<author>author@mywebsite.com</author>
<category>Category Name</category>
<pubDate>Fri, 11 Jan 2013 06:31:13 +0000</pubDate>
</item>



Please let me know how can I add image into RSS Feed vBulletin.