When you start a new skin in skinstudio, you will get the template of Diamond from Stardock Design. If you are modding for your own use, it's totally up to you how many images you want to replace with your own.
While I don't think you will have a problem if you upload a skin with a FEW default images in it, I would recommend that you try to make the skin as much your own as possible or it may not pass moderation.
