Just one thing, what happens if someone removes a skin from lfsworld while people are racing, I know the chances of this happening are extremely thin but for example I have a fair few public skins uploaded on my lfsworld and occasionally delete an old one to free up a skin slot.
As i say this will probably never happen but just curious as to how it works, would the person racing be specced mid race if a new person joins the server and triggers a request to download the now missing skin?
Its only the XRR and XFR (I think) that cannot have the roll cage painted any more, still works in the other gtr's.
As for the rear splitter, you are right you can't change it via a skin. It may be possible to change by editing the.dds it uses, but I'm not sure which file it is or if it is shared with other parts of the car/s.
The skins_x folder are the .dds files that lfs downloads when other users join so you can see there skin.
You should be saving your .jpg skins in the lfs\data\skins folder, your skins seem correct apart from the ones named XFGTR_skinname.jpg these need to be re-saved as XFG_skinname.jpg.
once you put them in your skins folder open lfs, go to the car selection screen and on the left you will see a list of skin names.
Just below this list is a button labelled "new colours" click that and it will list all your skins including the new ones, click one of the new skin names and then you will be able to select it.
Edit : I have renamed all the skins for you so they should work, .zip attached.