I'm sorry i'll have to describe it very vaguely because I can't remember exactly how to do it, but im pretty sure it goes something like this (this is the code you put when you get a big white text box to enter stuff in)
$vehicle_dff = include('C:\folder\where the\file is\vehicle name.dff');
$vehicle_txd = include('C:\folder\where the\file is\vehicle name.txd');
image_addfile('\models\gta3.img','$vehicle_dff');
image_addfile('\models\gta3.img','$vehicle_txd');
I believe that is the sort of thing I did, obviously you will need to change the part of the include( ) so that it points to wherever the file is on your computer, just play around with code like that and if you get it to work let us know how you did it. Hope that helps!