Well, if they are .JPG format, you should go to your LFS directory. LFS>Data>Skins (place .jpg here) or if its .DDS file, put it in to skins_DDS Go to pits, click on "new skin" and then it should compress (maybe idk) and then appear.
Found the problem... I was looking at the top of the list expecting it to be there, but the FBM skin I added was dated 2009 so was further down the list. Thanks though