Yes you can direct your computers' backup software to backup to the NAS instead of an external drive.
Yes you can store your photos onto a shared folder on the NAS. However, unless those photos are also stored on another computer, they will need a backup of their own.
Also note that since your PC and NAS are presumably in the same house, a fire which destroys one will probably destroy the other. Important data should be backed up off-site. Whether that's cloud storage or a hard drive/USB flash drive you take to work and store in a drawer is up to you.