I have began putting the source files, for all Palm skins and icons, in a directory on this site. The path to the source will be stated in the readme file of each download. I believe this complies with the GPL which states that the soure must be in the same place as the skin itself. The source will not be offered as part of the download in an attempt to keep small file sizes for each download.
As to what should be included as source, I do not believe this has always been clear for images under the GPL. I am operating under the assumption that it is the preferred form for making modifications to the image. Thus if the image files being created use layers, those layers will be included as the source. If the image file is only a BMP or JPEG image, that file will be included as the source.