Good tutorial for semi-realistic texturing for games?

Where can I find a good tutorial for semi-realistic texturing in Photoshop Elements or Gimp? I would like to be able to make textures like these.




What I am looking for is tutorial on how to texture a human completely by myself (i.e. If Possible, not using photos of real objects/people to make textures).
I am also looking for to make Eye textures, homemade.
Thanks.