Hi Xelados, thank you very much for your answers,
in pixeljoint i look at this Necky's comment : "I speak on behalf of us oldies. Our techniques have been aquired by studying scanned images when they were pretty rare to get hold of. I used to load up a photo in DPaint (when one was possible to get hold of, which wasn't very often) and study it to see how you could achieve the antialiasing, shading that you get from a photo." (
www.pixeljoint.com/pixelart/18762.htm)
it's great idea for learning texturing. pixel artists from Pixelation also use this technique ? or you have other methods ?