XML : Using PHP add xml node with double point?

I develop a XML sitemap generator, everything works well but I have problems with image node, this should be image:image, normal is my code so:

  $xml->addChild('lastmod', date('Y-m-d'));    

I try to add my image node so:

  $xml->addChild('image:image', '');    

If I create sitemap then I don't see but

How can I fix it to see node like image:image?

My prevous source:

PHP library for parsing XML with a colons in tag names?

http://www.php.net/manual/en/simplexmlelement.children.php

http://php.net/manual/en/simplexmlelement.addchild.php