$html = '<img id="12" border="0" src="/images/image.jpg" alt="Image" width="100" height="100" />';
$doc = new DOMDocument();
$doc->loadHTML($html);
$xpath = new DOMXPath($doc);
$src = $xpath->evaluate("string(//img/@src)");

Альтернативный способ:

$xpath = new DOMXPath(@DOMDocument::loadHTML($html));
$src = $xpath->evaluate("string(//img/@src)");

Решение в одну строчку:

$src = (string) reset(simplexml_import_dom(DOMDocument::loadHTML($html))->xpath("//img/@src"));