It's pretty straight forward, you can get them by proceeding through the main story.

These you can only obtain by making certain choices.
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To obtain these CGs you need to choose "Fight back" in the scene, which you gets after Jacob's invitation at night.
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This is just a cake walk(which takes ~20 minutes, but you can speed it up by using Cheat Engine or smth like this). To get this one you need to run out of time during your attempts to help Akira.
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So, this one leads you to achivment called "Endless Ending". To get this CG you need to choose "Send the previous cipher" or "Type in by intuition and send", when Ikeda is in Raymond's office.
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Pretty scary, huh? So, to get these you need to hide from the monster in the specimen room(although you can just run out of time and not choose something, you'll get the same CG's, but for some reason they won't show up in your CG library in game), when you're playing as Neneko.
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To obtain these CGs you'll need to hide from the monster in the filing room and choose "Hide in the locker", when you are playing as Neneko.
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And to get these CGs you'll need to choose the incorrect answer (so, no Riehl) , when you are choosing who is the "wizards" guy.

As I said earlier, you can get them by proceeding through the main story.

<work in progress>

I hope this guide was useful for you!
And always return to Shironagasu Island!