How to include enclosures

Is there an example on how to include a enclosure in the xml?
doing 
'enclosure' => 'url to image'

produces 
<enclosure>
//s3.amazonaws.com/test.s3.hotrodhotline.com/ui/5/02/5IdFF4SC1c.jpg
</enclosure>

is there a way to add the url inside the enclosure tag like this

http://www.w3schools.com/rss/rss_tag_enclosure.asp
