I assume what you want to do is replace several textures for gang members?
The modified textures/models are in the "models" folder of Betas.zip. Extract that somewhere on your computer, like Documents or wherever you store your mods.
With IMG Tool, you need to open it, then open gta3.img (GTA San Andreas/models/gta3.img), go to commands, and go to replace. Then you select each file in the folder you extracted with the mods. When you're done, go to commands, and click "rebuild archive." Wait for it to finish.
I would actually recommend using another IMG editor (such as this
one), because the IMG tool that came with the download doesn't allow you to replace or extract multiple files at once.