This simple script replaces Carl Johnson (for those who can't or don't want to use the skin selector) with whatever skin you like (the default is a scientist from Area 69).
To do this, take the skin that you need, give it the name wmosci.txd and wmosci.dff and replace it in the gta3.img file. Controls: Press the P key to turn on or off the replaced (press again) skin.