Originally Posted by wishie

Some programs that you can use are:
MSpaint
GIMP (free to download and has a layering system)
Pain.net (Free to download and has a layering system)
Graphics Gale (Not sure what the features are on this one)
Photoshop

There's other programs out there too that you can use if you don't like any of the ones listed above. You just need to make sure that you can lay down pixels and erase pixels.