In Override von mod_menu/.../default_component.php

$Query = $item->query;

if ($Query['option'] === 'com_content' && $Query['view'] === 'category' && $Catid = (int) $Query['id'])
{
	$db = JFactory::getDbo();
	$query = $db->getQuery(true);
	$query->select($db->qn('created'))
	->from('#__content')
	->where($db->qn('state') . ' = 1')
	->where($db->qn('catid') . ' = ' . $Catid)
	->order($db->qn('created') . ' DESC');
	;
	$db->setQuery($query);
	$lastDate = $db->loadResult();
	
	$lastDate = strtotime($lastDate);
	
	// nicht älter als 2 Tage
	$intervall = 2 * 24 * 60 * 60;
	
	if ($lastDate > (time() - $intervall))
	{
		$item->menu_image = 'images/powered_by.png';
	}
}