<?php

$html = <<<HTML
<div id="res">Some text inside DIV
<img
class="size-full wp-image-1561 "
alt="Class-A warehouse facility developed by Panattoni Europe in Germany for Rudolph Logistik Gruppe."
src="http://e...content-available-to-author-only...e.com/wp-content/uploads/2012/12/Gruppe.jpg">
<img
alt="Class-A warehouse facility developed by Panattoni Europe in Germany for Rudolph Logistik Gruppe."
src="http://e...content-available-to-author-only...e.com/wp-content/uploads/2012/12/Gruppe.jpg">
</div>
HTML;

$dom = new DOMDocument();
@$dom->loadHTML($html);  // Using @ to hide any parse warning sometimes resulting from markup errors
$dom->preserveWhiteSpace = false;
$images = $dom->getElementsByTagName('img');
$imgs = array();
foreach($images as $img) {
  	if ($img->attributes->getNamedItem("class") != null) {
        $imgs[] = $img;
   	}
}
foreach($imgs as $img) {
    $img->parentNode->removeChild($img);
}

$str = $dom->saveHTML();
echo $str;