Go to LFS world > My online car skins. There should be a link to increase your skin-upload-slots. That link will redirect you to buy skin slots.
If I recall correctly, its 50 skins slots for £1.
PS: I think the minimum payment is £3, so you cant just pay £1 for 50 skin-upload slots. Best option is to buy S1 (where you get 25 skin upload slots anyway), but if you're hard on cash and still want to upload you can do so for £3 - £1 for 50 losts and the remainig £2 will be in your account, which you can use later to buy a license/more slots/high-res downloads etc.
Have fun.