Me and manuel have had a discussion about enabling uploading of user contributed skins within the nearest future. However, before doing so, we need a better documentation page. What we currently have on making skins is this:
http://www.makehuman.org/node/127
If someone could find it in them to contribute a few lines of text on the topic, this would be very helpful.
A skin is here a texture such as what is included in the MHX exports. For the vanilla skin, see http://www.makehuman.org/node/124