Huge, but simple bug affects structure error:

This is wrong:

if(!empty($permalink)) {

    foreach( array_keys($imgurls) as $imgurl ) {
        $xml .=
            "<image:image>" .
            "<image:loc>" .$imgurl ."</image:loc>" .
            $license .
            "</image:image>";
    }
    $xml .= "<url><loc>" .remark_escape_xml_entities($permalink) ."</loc>" .$img ."</url>\n";
}

It should be:

if(!empty($permalink)) {
    $img = '';
    foreach( array_keys($imgurls) as $imgurl ) {
        $img .=
            "<image:image>" .
            "<image:loc>" .$imgurl ."</image:loc>" .
            $license .
            "</image:image>";
    }
    $xml .= "<url><loc>" .remark_escape_xml_entities($permalink) ."</loc>" .$img ."</url>\n";
}