SolarLune is a indie game developer (Blender, GIMP, Python) he's doing a serhttp://c55.me/minetest/forum/viewtopic.php?id=697ies of tutorials on pixel art on GIMP:
Pixel Art Tutorial 1 - Torch
Pixel Art Tutorial 2 - Perspective
Pixel Art Tutorial Series - Part 3 - Light and Shadows
Pixel Art Tutorial Series Part 4: Color Contrast
Pixel Art Tutorial Series - Part 5: Simple Animation - Run Cycle
Others tutorial posted: Pixel Art : Sword in One minute with The Gimp
[PhotonBlog] 16×16 Pixel Art Tutorial:
http://minetest.net/forum/viewtopic.php?pid=22116#p22116
How to... make textures with minecraft style (by Shelby Smith)
http://minetest.net/forum/viewtopic.php?id=855