Either the general discussion thread got moved, or you found the right place
If you just want new interiors, look around. There are many that have been released and you`d simply need to replace the files in your lfs data/dds folder with the new ones (after backing up the originals first). Some older interiors no longer work under patch Z tho, so be careful.
If you want to try making your own, you`ll need something like paint.net, the gimp or preferably photoshop, but that`s not worth buying unless you really know what you`re doing and can afford it. Either of the other two will do the job fine for playing around.
Name the car you fancy trying, your version of LFS and I`ll point you to the files.