D/L a evaluation copy of Paint Shop pro - you will find it easier to work with for doing forza decal work - you can then use the decrease colour depth to 256 colours (8 bit) and then save as tga.
I use a combination of PS and PSP to do my cars
I use PS for the original sizeing and composition of images and then move over to PSP to convert to 8 bit and save as TGA (performing any haltone cleanup before saving)
I also use PS for custom vinyls, as they turn out better than PSP