For anyone struggling with unlocking the gallery, I've compiled all the spawn locations for the magazines that I've come across into an imgur post.
This is not an all inclusive list. I may have not seen every spawn, and the magazines stop spawning after you find all 16.
From my little bit of time researching it in game, it seems to be a good 75-80% of the time at least one magazine will spawn in at least one of the rooms, but there will be times where there isn't a spawn (at least from what I've been able to find. I may have missed a magazine spawn in a new location on these nights)
There CAN be multiple spawns in the same run, and you CAN have multiple magazines spawn in the same room. I had one run where I had all three of the valve room magazines spawn in, and another run where 2 of the 3 power room magazines spawned. As well as runs with two spawns in two different rooms.
The only room that I didn't see a spawn was the long hallway that the pink fox spawns in on night 12, but again, I may have just missed it one night.