Hey Creator of this mod...
I've 2 questions.
First: My minecraft just crash every time I want to join an Single player world. Why is it so?
And Second: where must the PNG with my skin in?? By uuid or in the skin folder?

Sorry about the late reply.
There should be crash reports in crash-reports folder, the information in them show why the game crashed.
Either way should work, skins folder accepts files named after username, uuid folder accepts files named after uuid.