Prova questo
Codice PHP:
<?php 
$doc = new DOMDocument();
$doc->load('images.xml');
$xpath = new DOMXPath($doc);
$root = $doc->documentElement;
$query = "/root/image[filename/text()='image_1.jpg']";
$image = $xpath->query($query);
$oldImage = $root->removeChild($image->item(0));
echo $doc->saveXML();
?>