it just increases your max fullness. That’s all it does. Once you use it, it should increase max fullness and be consumed after that.
Check the computers in the lab in the basement of the Castle.
I have to ask is there a new game plus? as that would be the only way to fill out the gallery right? I would like to unlock the full gallery if possible. a guide to get all the different events for the pictures even the special one would be the next best thing. I noticed that in the gallery there seem to be one more pic for Eira & the Two slime girls to unlock of course I can only guess as there is no hint to say if they have more to unlock is or not. that could also help lead the player to keep searching and playing for more of that sweet goodies that are hinted at in the game.
There isn't at the moment. Generally some pictures won't be unlockable in one playthrough. I did think of adding in a new game plus. But that would be an undertaking on it's own (since it requires a little bit of planning and I actually had an idea of how I wanted to do it as well)
It will come eventually, but not just yet.
There is one more glitch that I found Brittany 3rd weigh gain picture does not appear in the menu screen but it shows just fine in the mirror. And this last one may be a glitch? Her sister Tiffany gains weight faster then her sister and it seems faster everybody else as well just to let you know so you can take a look if you want.