I just updated to Magento 2.1.6 on Windows and had this same issue. I did a little searching and came across a post on Github here.
In a nutshell you need to do two things in vendor/magento/module-catalog/Model/View/Asset/Image.php.
The first thing is replace all occurrences of DIRECTORY_SEPARATOR with '/'
The second is edit the getRelativePath function to look like:
private function getRelativePath($result)
$prefix = $result == '/' ? $result : '';
$result = $this->join($result, $this->getModule());
$result = $this->join($result, $this->getMiscPath());
$result = $this->join($result, $this->getFilePath());
return $prefix . $result;
You need both changes or it won't work. Just replacing DIRECTORY_SEPARATOR still throws a forbidden error. Not including the $prefix elements in getRelativePath causes the forbidden error to go away, but product images won't load.