It's linked in the post as well, but more info at:
Convention Game - Photo Challenge | Halo Costume and Prop Maker Community - 405th
Basically the actual game is you roll a d100 or use a random # generator 7 times and get your numbers (any repeats == reroll). There's a list of 100 photo prompts that you match up your rolls with the prompt of those numbers.
For example if you were to roll a
14 -> 405th member(s) sleeping
49 -> Halo: Memorial firing squad (e.g. Halo 3 end credits)
10 -> Group Photo, Rockette line or Red Rover
38 -> Halo: Spartan trading weapon with ODST/Marine
and so on.