Hello there, welcome to LFS!
Most "how-to" stuff and all sorts of info regarding LFS can be found in the LFS Manual.
Have a look here for skin-upload info:
http://en.lfsmanual.net/wiki/S ... upload_skins_to_LFS_World
Make sure that the skin file is properly named. File starting with three-letter car code and "_" and ending with ".jpg" (not ".jpeg").
i.e. FXO_Speedy77.jpg
And as stated in the manual the size shouldn't be more 1024*1024...
PS: You should try making your own Coca Cola skin; its fun to race around in a skin you made yourself
(but maybe that's just me)