You can edit the interior picture files (*.dds) by using some converter to get them in *.jpg or *.bmp or *.whatevah format. Then just edit the files in your pic editor and convert them back to *.dds and throw them back into your LFS\data\dds folder. Remeber to backup any files you edit.
You can't edit the shape (3d-models) of the cars, nor inside or outside. No one else can't see your interior skins. And all the same model cars will have your interior skins. Also changing any XR interiors will change the XR gt, XRR and XR turbo interiors.
If you want to make "outside skins", the templates can be found in your LFS\data\skins folder. Just name in correct way, like LX6_myskin and upload it to LFSW and it'll be fine.