https://3v4l.org/Cai40
$dom = new DOMDocument();
$dom->loadHTML($html);
$xpath = new DOMXPath($dom);
$nodes = $xpath->query('//img[contains(@class, "popup-img")]');
foreach($nodes as $node) {
$a = $dom->createElement('a');
$a->setAttribute('class', $node->getAttribute("class"));
$a->setAttribute('title', $node->getAttribute("alt"));
$href = parse_url($node->getAttribute("src"), PHP_URL_PATH);
$a->setAttribute('href', '..' . str_replace('/thumbs/100/', '/', $href));
$a->setAttribute('style', "background: url('..$href'); -webkit-background-size:cover; background-size:cover;");
$node->parentNode->insertBefore($a, $node);
$node->parentNode->removeChild($node);
}
echo $dom->saveHTML();