OK, I solved the issue but using a regex (thanks to this page where the issue was discussed, too):
// retrieve link to condition image - in description element
$xml = simplexml_load_string($weather_feed, 'SimpleXMLElement', LIBXML_NOCDATA);
$description = $xml->channel->item->description;
//preg match regular expression to extract weather icon
$imgpattern = '/src="(.*?)"/i';
preg_match($imgpattern, $description, $matches);
$aExport['img_url'] = $matches[1];
Cheers