The skins_x folder are the .dds files that lfs downloads when other users join so you can see there skin.
You should be saving your .jpg skins in the lfs\data\skins folder, your skins seem correct apart from the ones named XFGTR_skinname.jpg these need to be re-saved as XFG_skinname.jpg.
once you put them in your skins folder open lfs, go to the car selection screen and on the left you will see a list of skin names.
Just below this list is a button labelled "new colours" click that and it will list all your skins including the new ones, click one of the new skin names and then you will be able to select it.
Edit : I have renamed all the skins for you so they should work, .zip attached.
can you post up a screen shot of the error given by lfs?
Be careful un installing and reinstalling/unlocking s2 as you only get 3 unlocks, once you use them you will have to wait to get a new one (one every friday until you get to the max of 3 again) or have to request more from the devs.
just download a fresh install of lfs from http://www.lfs.net/?page=downloads. Install it in a different folder and copy your sets, skins etc from your old install into the new one
Similar to Tims post just a different way to do it
In Gimp (not sure about ps) i do the base colours on one side then hide the blackmask and shadowlayer, select the side i've done, Right click on the selected area and select "copy all visible" which will copy all visible layers in your selected area then unhide the blackmask and shadow layer, paste the side, flip it and move it to fit.
If you click the link posted by madcatx and click the second from bottom link on that page you can email the devs directly who will sort it out for you.