How do I transfer my iPhone photos/videos to a NAS with afp-protocol?

I was looking for this too. In Image Capture, I think you should be able to choose Import To: Other (if you scroll to the bottom of the choices there), and then choose your NAS storage location. At least I am able to do it that way. My NAS shows up on the left in the popup dialog similar to how it shows in Finder. Anyway I still prefer to copy to my computer first, and then to the NAS, so that I am sure I get all my photos without some error.