Hello,
You can use Indexer, or getDocxPathQueryInfo with a custom XPath query to get that information:
$referenceNode = array(
'customQuery' => '//w:drawing//wp:docPr',
);
$contents = $docx->getDocxPathQueryInfo($referenceNode);
foreach ($contents['elements'] as $content) {
echo $content->getAttribute('descr');
}Regards.