[Ash.]

Try installing the game to a different directory. For example, instead of installing it to "C:/Program Files/Rockstar Games/GTA San Andreas", try installing it to "C:/Program Files/GTA San Andreas". Windows 7 stores thing in a 'cache', and this 'cache' now probably contains your modified gta3.img file. Installing it to a different directory will make Windows think that the gta3.img file is different (which it is, but while it still exists in the normal installation folder it won't check for changes unless YOU modify it). Installing it to a different directory (a different partition, would also work) should fix the issue you are having.