If you follow the following sequence, it should work...
1. Copy your current iPhoto (or Aperture library for that matter) from (most likely) ~/username/Pictures/ to your Synology.
2. When you're all done copying, making sure that iPhoto is closed. first press the alt/option key on your keyboard and then launch iPhoto. You will be presented with a window asking you "Which photo library do you want iPhoto to use?". Navigate to your Synology and locate the file you just copied et voila.
3. Once you have confirmed that all is fine, you may wish to DELETE the original library from your user folder : it will prevent confusion. However please note that a NEW EMPTY iPhoto library will be created in your user folder - just ignore it.