I'm not sure if I should ask this question on this thread or not, but can someone pls tell me what does "CG" stands for? (it's in the Extra)
It's not an extra
CG as in Computer Graphics, simply stands in as an umbrella name for the pre-rendered images used by certain game engine, an example would be Renpy, Unity, etc.
And depending on the type of game engine you are using, you can use certain software to extract the pre-rendered images.
Example
Unity: UnityEX with DDS Converter or Unity AssetStudio.x64.v0.9.0.27
Note - You can't extract images from real time 3D games like The Twist)
Renpy: Unren, RPA Extractor for Windows, RPATool
RPG Maker MV: RPG-Maker-MV-Decrypter-master
RPG Maker VX Ace: All decrypter/RPG Maker Decrypter