Free- Gimp
www.gimp.org/
-Use hard brushes
-Use Hue/Saturation/Curves/Levels/Colour balance for colour correction.
-Offset or use the make seamless filter to generate seamless tiles.
-Work on different layers and then download the export layers to image action
<img src="http://fc01.deviantart.net/fs41/f/2009/009/a/c/Armoured_Figure_Sprite_by_Dsarchy.png" border="0">