Surprisingly not many people are aware of what mod loader is or how to use it. Pretty much, you can use mod loader for skin, weapon, sound, and vehicle mods. Instead of going into the GTA3.img, you can simply just drag the files into your mod loader folder located in the root of your GTA SA.
First things first, click the link below to download mod loader.
http://www.gtagarage.com/mods/show.php?id=25377
Once you have downloaded mod loader, you will need to extract the zip file containing the required files. Once you extract the .zip, you will need to drag all of the contents to your GTA SA root folder. That is pretty much it. Once you download a mod you just need to drag the files into the mod loader. It is suggested you make several folders in mod loader (Cars, Sounds, Weapons etc) in order to keep it all more organized. Keep in mind, you can also add mods while in-game using this, but very rarely it might crash.
Mod Loader Creator: Link2012