TXDworkshop. Open gta3.img file with it ( its in ur SA folder / models ) and then there are the files , for example nrg-500 , scroll down until you fing nrg500.txd , then there are body , metal , brakes and decals , export nrg500body in PNG and then paint or whatever it in some photo editing program , then save as .png , import it back in the nrg500.txd file , save and you should be done .. , BTW there is somewhere a tutorial here created by SlyFox