If you open an image from within SkinStudio that is a .tga it converts it to .png in you graphics editor. If you decide to save that image as a .png somewhere else for use maybe on a future skin, it will save it as xxxxxxx,tga,png, When you import that image back into a skin it will bring it in as xxxxxxx.tga.tga. So whenever you save an image brought in from SkinStudio to as a .png, you need to remove the .tga from the title. Don't know if this is your issue, but it has happened before to me.