(From left to right, up to down, and reference followed by codes)
Come get some friendliness pellets!- undertale, flowey
Is Herobrine real?- minecraft, herobrine
Four nights at Frank's- fivenightsatfreddys
The world needs heroes- overwatch
Hunted and alone... - inside
Hello, Two Forks Tower!- firewatch
Everything sucks forever- nightinthewoods
Those dogs sure like to party... - va11halla
Please reboot with hypnOS- hypnospace
Lunar Transfer Station- tacoma
Build, sail, repeat- makesail
The only verb that matters.- littleredlie

(Left to right, up to down)
Top of the mornin' to ya!- jacksepticeye
How's it going, bros?- pewdiepie
Hello, everybody, my name is...- markiplier
Producer slash seducer- kindafunny
Vape naysh yall!- vapenation
What we do here is go back- idubbz
I'm here with the new game Skyrim- videogamedunkey
Curious Gamers- grilledchaos
The Legend of Grime- grimecraft
Minecraft derp- ssundee
Fala galera, beleza. Eu sou...- ocellbito
Yo tout l'monde c'est...- squeezie

xBurpx it's a s-se-secrett- rickandmorty, morty
Shut up fat ass!- southpark, cartman, eric cartman
Number one in the hood- aquateenhungerforc
Mathematical!- adventuretime, finnthehuman, finn, jakethedog
Josh Desano- kappa
Nyanyanyan- nyancat
Such secret, very confidential- doge
Moaning intensifies- kreygasm
What's a Flash Portal?- newgrounds
Mondo Media- happytreefriends
The Burninator- trogdor
(The last one is a hidden Rick Roll icon; you get it through gameplay, and there are many guides on how to get this already. There is no code.)

Anyway, that's all of them. This is my first guide, so don't be a jerk about its quality. As I said in the description, though, feel free to comment on any additional codes you know of and to what icon they go to. I'll add them. :)