ok considering we've got a possible lead with the bistro: "If you're an administrator enter your 5-digit PIN", I'm thinking the smaller pieces form a 'puzzle' containing 5 numbers.
with 18 unique pieces, that's either a 2x9 box, or a 3x6 box... obviously the latter is more feasible. I don't think the missing sms images will reveal any new smaller pieces (but let's hope they come in anyway, cuz ye never know)
so print the 18 small pieces, and see if you can rrange them to get 5 numbers! Looks like they'll be overlapping.