The script.img is binary type file located in the same place as the main.scm; San Andreas/data/script
This file holds all the external scripts, of the main.scm after you compiled you're main.scm,
if you downloaded a main.scm mod with external scripts, without the new script.img included, the game will crash, when trying to play the script; e.g Basketball
If you're main.scm contains no extranal scripts within it, then there is no need, for the file, when playing the game.
It will contain just a dummy header:
When you compiled a main.scm file with Sanny Builder, it will write the necessary script.img for it, if you do not change, any extranal scripts, you may not need to replace, the old one with the new one.
The script.img stays in the games memory until the game ends, you can not move and/or modify it while the game is running.