^ the file ID being the same as DMM aside, you can actually find them if you open up and read the text in 'scenario.json'
i.e. If you want Artemis's file ID, just open up her scenario.json and you will very soon find 5068-2-2_a in the scene text. Also note that, you might wanna add _h to access the slightly higher quality file even though the game only loads the LQ one. So that aforementioned Artemis scene CG file would be 5068-2-2_a_h.jpg