No, there's no supported means to target archival storage onto a Windows box with Time Machine.
As you're likely aware, Time Machine only supports specific devices as targets, and the file server are currently OS X Server, Time Capsule devices, and the most recent generation of AirPort Extreme devices with attached disks. Not Windows Server boxes, Linux or other gear.
More than a few folks and a few discussions and various hackery have allowed random devices to appear to work (and it's possible your NAS device has seen this same hackery), but whether those configurations will continue to work or will react appropriately during the inevitable network and device glitches is very far from certain. Among these sequences, whole-disk installs and related system restorations are key, but are also something that doesn't necessarily get thoroughly tested.
A Mac Mini running OS X Server can host a fair amount of connected storage, if you decide to replace or supplement your NAS.
For more information on Time Machine, start reading here.
It is possible to roll your own backups and to use third-party tools to perform the backups onto remote devices, and one of the more common recommendations here are based on a command line tool known as rsync. Copy your files over, and either acquire or custom-create your own archival processing. (If you're interested, search for previous discussions of rsync and backups for some of the available options, and there are tools and scripts available. rsync is pretty ugly to get going initially — it's arcane — but it works pretty well, and various folks have already figured out the syntax and posted example scripts.) Carbon Copy Cloner is an alternative to Time Machine, and there are others.
On the subject of creating your own custom software for these and related tasks, it's also possible to use software that targets photo studios and their related processing, and that manages the photos and videos and other assets, and that can manage backups and related processing, and various of these will deal with reprints and purchases and related. That's a bit further along than a backup device or a Time Machine backup, but it's something that might replace the requirements. (You probably don't care nearly as much about OS X and most other stuff here as that can be re-downloaded or reinstalled, it's your photos and your customer data that are key here, and that are not replaceable. That stuff is the core target for your backups.)
