ok well, back when i had my old PC.. i wasn't able to add new txd's in my game, only dff's n such (cars) but after a lot of asking and no answers i just started trying things when i found this... which is pretty obvious now but at the time i had no idea what it was lol (txd.img) so after some testing i came up with this.
*origonal text VVV*
wel a while ago when i started stunting in vc on pc i had a shitty ass pc that would crash when using fraps 
check my uss audition from adges ago on my youtube page the pc version...but, no 1 knew why i couldnt install new txd's of cars and things...the gta3.img had all the right txd's in it how ever because the pc was so shitty it had created a txd.img for the txd's for some reason, which the game reads most txd's from, so for all those people with old pc's like my old 1 who want to install skins or w/e into your game, what you need to do is1: delete and add the dff (for models) in the gta3.img of the vehicle you want to replace
2: open the txd.img which is in the same folder...(models) and delete the old txd and add the new txd
3: open the carcols.dat in your data folder and find the line that is for the vehicle your replacing and just highlite the line and paste the new line it
(only for downloaded models...recommended you don;t do this with stunting)
(for people who just want to add skins step 2 is all you need to worry about)
for people who allready knew this before, screw you because no 1 helped me, i had to figure it out myself
hope i helped you and enjoy your skins
