The main thing here, is that the PHP on your server needs to be able to read all images as if it is on a local disk. So for that to happen, the NAS needs to be setup as part of the servers storage device(s). You can't have a remote server look for images inside your local-hosted NAS dir ... Even if you managed to map it somehow, which is unlikely, it would be incredibly slow for the server to "read" data from a remote disk.
Having said that, I know it's possible to setup the entire web server on NAS with PHP and storage. In this case, you won't be using a "Windows web server" also, but just the NAS itself. Also, if you are intending to run a public website, you would need to use some dynamic DNS service to map your domain to your internet IP (I assume the NAS will be in your home). Tedious, but possible.
The cheapest "pro" solution you would find, would be to setup a $5.00/month server at
vultr.com, and expanding storage with HDD (cheaper than SSD) for $1.00/month per 40GB. This requires you to manage the web server yourself though (eg install Nginx or Apache) through SSH.
Linking your home NAS drive to your web server in a different physical location is not going to work. Although you can host standalone images on cheap/external storage or Object Storage (like
Amazon or
Vultr $6.00 1TB), this won't normally work because it's not part of the server/PHP file system.