top left of the setup screen under car colours click add for a new colour then change the tab to colours and down the bottom you can choose the texture you want to use. i hope that made sense :P
Dont quote me on this but I think they need to be jpg for lfs.
PSD are the 'build files' people like to use because of th layers. but ultimately you export out the final skin as jpg for lfs to the skins folder.
Of course, keep that psd handy for revisions later.
I have problem with helmets on X patch... I made helmet and I wated to add it in LFS and I put it in data/skins/, start LFS and can't find it ingame. Can someone help