Hi there,
I think the best and native way to back up iPhoto is using Time Machine.
Here is a course about using Time Machine with ASUSTOR NAS
Time Machine: Best Practice.
Another choice is manually copy iPhoto library into NAS on regular time schedule.
Normally the library is under "/Users/your name/Pictures/iPhoto Library".
Or you can find it as below steps:
1. Quit iPhoto if open.
2. Hold down the Option key as you open iPhoto.
3. A windows will appears, and you can see the path.
Please let me know if that works for you.
Cheers,