I'm gonna recommend a program I found while browsing the internet a few days.
http://www.humanbalance.net/gale/us/
The original software is Japanese, but when you download it, it'll be in english.
GraphicsGale is a program tailored to sprite artists, and many of it's features are for sprite artists only. I love the interface, and the simplicity of it all, but the one thing that's weird about it is that you can't open up .gif formats in it.
The best part about it, aside from all the awesome features it has to offer, is the fact that it's free.