For 2 users storing family photos, my guess is that a 32gig SD card would be fine. This is mostly on general grounds: 16 gigabytes is just a touch on the low side to store an operating system plus some software, and last I checked 16 gig cards are like $7 and 32gig cards are like $9 (at least in my part of the world). Extra storage is rarely a bad thing and the price difference between the two options is really modest.