instead of replacing the alt tag you could add-an-attribute-to-a-tag
function add_lazyload($content) {
$dom = new DOMDocument();
@$dom->loadHTML($content);
foreach ($dom->getElementsByTagName('img') as $node) {
$oldsrc = $node->getAttribute('src');
$node->setAttribute("data-original", $oldsrc );
$newsrc="".get_template_directory_uri().'/library/images/nothing.gif';
$node->setAttribute("src", $newsrc);
}
$newHtml = $dom->saveHtml();
return $newHtml;
}
note: i didn’t quite tested this code, so be careful 🙂