Hi MikeJohnson,
Just because you have replaced your gta3.img does not mean you will remove every mod, depending on what mods you have used, as they may rely on other parts of the install.
The Answer:
I personally would uninstall everything to do with GTA SA (sad but true) this also includes everything that you have contained inside your Documents folder, as it is (under your Rockstar saved games). After you have uninstalled everything so there is no trace. I suggest checking the install folder manually (C:/program files. . . . ) and making sure there are NO files in that folder! its possible that even after the install if you have any third party mods you put in that the Rockstar installer Won't Remove Them!
I normally remove the whole folder then restart.
Once this is done you can begin reinstalling and as soon as that is done check whether or not the mods are there!
Good luck with you venture.
Happy Gaming!
Regards
DellStorm