So, I did a skin for the XRR car on paint.net and saved it, but when I hop in LFS this error pops up saying that it's "impossible" to load my skin
what do I do to fix this?
any help appreciated

It doesn't matter where you made skin. Just make sure to name it XRR_somename and place it in skins folder. The file type should be jpg and image resolution 1024×1024px. If you want other players to see your skin, then you additionaly have to upload it to lfs world.