Question about background images

So, I’ve noticed that a lot of other Choicescript games (Choice of the Dragon, for example), have background pictures which go behind the text. Seeing as I’ve seen some player-made stuff with the same kind of thing (Ruler of the Red Empire, for example), I was wondering if anyone was willing to tell me how to put that into my own game.

I’ve been wondering about this too, I can only seem to change the color.