I just looked at the manual for your NAS and there is no indication that what you want to do is possible with this box. It looks like RAID 1 is an all-or-nothing option.
While I'm sure they exist, I don't know of any particular low-cost NAS that offers this functionality. You could build your own NAS using a motherboard that supported multiple RAID levels on the same drives. Intel's RAID controllers support this. However, it would be more expensive than an appliance like the one you've purchased.
^ this plus be aware raid 1 is NOT a backup in any way shape or form. It's about trying for better availability in case of a failure but you'll still need another drive to act as the backup of the raid array.
If you perma-delete a photo in raid 1, it's still gone - unless you have it on your backup drive.
I just want to be able to add photos to a folder and have them automatically copied to another drive so if the first drive gets corrupted then I haven't lost them all. I was under the impression that raid 1 did that?
Sometimes. It's also possible the entire array can become corrupted.
It's always best to have an external drive in case of a power surge. But, it may be possible to use Acronis or Easus Todo (or whatever the NAS comes with) to schedule automatic backups from one drive in the enclosure to another.
RAID 1 is strictly a redundancy scheme, not a backup scheme. There is a huge difference.
For instance, if your OS or controller starts writing bad data to the primary drive and destroys it in the process, that same bad data will be written to the mirror. A backup, on the other hand, is a selective or automatic process to a separate drive (preferably on a separate controller) that backs up your data.