Windows does have a built-in Backup and Recovery System that can be very handy, but it doesn't create ISO images. ISO is just a file type for creating images of CDs and DVDs but is not really made for complete computer system images.
Putting in a NAS device is incredibly handy for any business, but it's not the NAS itself that gives you the ability to back up computers. That's all done at the actual computer system, the NAS is just storage connected onto the network that you can use to actually save those files to. You can manually go and configure each computer to automatically back up to the NAS but there are some key points to consider here.
1) Your backup space is all fair game for Windows Backup and Recovery. What this means is if you have a single partition set up on your NAS (which is most common) then Windows doesn't care if you have other stuff saved on that drive or what sort of usage you need. It's going to continue to write files and backups to that partition until it is full. This means if you're using that shared storage partition for use such as customer files, documents, etc. then you're going to be limited in overall capacity for such files as Windows will start taking up a lot of space on that partition for each of your computers backing up.
2) Windows Backup and Recovery is dependent on underlying hardware, and isn't always bulletproof. This means if you have a computer that goes down and have to replace to different hardware, your system recovery images won't work anyways, or at least it's incredibly unlikely. System images also copy the necessary drivers within Windows to get your computer back up and going, so if the underlying hardware isn't the same, your drivers won't work and simply the recovery will not boot and operate properly. I've also had problems in the past with Windows somehow not recognizing that there are any previous backups on a storage device. Simply, even though all of the files for that specific system backup were right there on the hard drive, Windows recovery CD wouldn't recognize that anything usable was on it.
3) This is an computer-by-computer configuration. Windows Backup and Recovery, like many others such as Acronis, are set up and operate on each computer independently so that means you have to manually go and configure each backup individually and ensure all are working individually.
You can look at stepping up from a NAS device to an actual server to address some of these things. Windows Server 2012 Essentials R2 features many great built-in utilities, and the one I like the most is their backup manager. You can join computers to the server, and it will automatically configure and take care of system backups onto the server, and you can even set up individual drives or partitions for this storage role. You can adjust individual computers or all computers simultaneously. And while the full system image recovery IS dependent on the underlying hardware, you can actually go and access and recover individual files on each computer back in time as well, not just the entire system image. I believe you can even inject your own additional drivers so that the system can be restored to different hardware.
Going with a full business server versus a simple NAS device is going to be more costly, yes. But there is a lot more flexibility there. Additionally the server itself can utilize standardized desktop/server hardware which can be easily upgraded or replaced, which is not the case with a pre-built simple NAS device.
